**Gamma Rays In Space**

[39] Burt C. Plastic Scintillation Spectrometry. PhD dissertation. Southampton University; 2009. Available from: http://eprints.soton.ac.uk/72351/1.hasCoversheetVersion/Thesis.

[40] Corre G, Boudergui K, Sannié G, Kondrasovs V. A Generic Isotope Identification Approach for nuclear instrumentation, In: IEEE proceedings of Advancements in Nuclear Instrumentation Measurement Methods and their Applications; 20–24 April

[41] Hamel M, Dehé-Pittance C, Coulon R, Carrel F, Pillot P, Barat É, Dautremer T, Montagu T, Normand S. Gammastic: towards a pseudo-gamma spectrometry in plastic scintillators. In: IEEE proceedings of Advancements in Nuclear Instrumentation Measurement Methods and their Applications; 23–27 June 2013; Marseille. IEEE; 2014. doi:10.1109/

[42] EJ-256 data sheet. Available from: http://www.eljentechnology.com/index.php/prod-

2015; Lisbon. IEEE; 2016. doi:10.1109/ANIMMA.2015.7465628

ucts/plastic-scintillators/ej-256 [last accessed 2016-07-26].

pdf [last accessed 2017-01-05].

66 New Insights on Gamma Rays

ANIMMA.2013.6727889

#### **Chapter 4**

### **Gamma Rays from Space**

Carlos Navia and Marcel Nogueira de Oliveira

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67176

#### **Abstract**

An overview of gamma rays from space is presented. We highlight the most powerful astrophysical explosions, known as gamma-ray bursts. The main features observed in detectors onboard satellites are indicated. In addition, we also highlight a chronological description of the efforts made to observe their high energy counterpart at ground level. Some candidates of the GeV counterpart of gamma-ray bursts, observed by Tupi telescopes, are also presented.

**Keywords:** gamma-ray astrophysics, cosmic rays, particle detectors

#### **1. Introduction**

Gamma rays are the most energetic form of electromagnetic radiation, with a very short wavelength of less than 0.1 nm. Gamma radiation is one of the three types of natural radioactivity discovered by Becquerel in 1896. Gamma rays were first observed in 1900 by the French chemist Paul Villard when he was investigating radiation from radium [1].

They are emitted by a nucleus in an excited state. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus. Gamma emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

On the other hand, cosmic-ray particles (mostly protons) that arrive at the top of the Earth's atmosphere are termed primaries; their collisions with atmospheric nuclei give rise to secondaries. These secondary particles are constituted by pions (subatomic particles); the dominant decay of a neutral pion is the electromagnetic decay in two photons (two gamma rays). This process is the origin of the highest energy gamma rays.

Nowadays, the artificial production of pions (and consequently gamma rays) in N-N collisions is produced copiously. The artificial production of pions started in 1948, with Lattes and Gadner using the 184-inch synchrocyclotron at Lawrence Berkeley National Laboratory (California) which accelerated protons to 350 MeV [2, 3].

As gamma rays coming from space are absorbed by the Earth's atmosphere, the first detection of gamma rays coming from space was through satellites. Indeed, the Explorer satellite in 1961 confirmed the existence of gamma rays in space [4]. Gamma rays coming from space have frequencies greater than about 1018 Hz; they occupy the same region of the electromagnetic spectrum as hard X-rays or above them. The only difference between them is their source: X-rays are produced by accelerating electrons, whereas gamma rays are produced by atomic nuclei decays and/or nuclear collisions.

There is a large variety of gamma-ray sources from space: the most important is the Sun's transient activity, such as solar flares and coronal mass ejection (CME). This means that there is also a "gamma-ray background" due to other stars in our galaxy and from the stars of other galaxies, as well as that expected from the interaction of cosmic rays (very energetically charged particles in space) with interstellar gas. There are also sources of gamma rays with continuous emission, or long-lived gamma-ray emission from compact sources. This emission is in the high-energy region from GeV to TeV. The Fermi Large Area Telescope (LAT) produced an inventory of 1873 objects shining in gamma-ray light [5].

The most intense are the galactic objects, such as the Crab nebula (Taurus constellation) and the W44 nebula (Aquila constellation), both two supernova remnants. There are also bright extragalactic objects, such as the objects called active galactic nuclei (AGN); the most bright are the so-called Markarian 501 and 421 AGNs. These extragalactic objects were discovered by a ground-based gamma-ray telescope, the "air Cherenkov" Wipple telescope [6], confirmed by telescopes mounted on satellites.

Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe: they are bright flashes of gamma rays for a short period of time, in most cases less than 100 s, with a photon's flux of around 0.1–100 ph/cm<sup>2</sup> /s/keV at Earth. There is some evidence that indicates that a GRB of long duration (i.e., above 2 s). They are associated with exploding massive stars called as Hypernovas. The signature of these explosions is two opposing beams of gamma rays and if any one of these beams is in the Earth's direction, we will see the gammaray burst.

In addition, sometimes, several narrow lines are detected; this radiation comes from the material around the explosion that was excited by the blast and that permits the detection of its host galaxy. However, sometimes, no narrow gamma-ray lines are detected, especially in GRBs of short duration (less than 2 s), meaning that some of these bursts come from different progenitors, such as merging compact objects such as black holes or neutron stars, or perhaps from a massive star explosion, but without the formation of a supernova.

In most cases, the gamma rays from GRBs, observed by space crafts, have energies in the keV region; a fraction has photons in the MeV region, called a high-energy counterpart. In some cases, photons can reach energies up to several dozen of GeV. In this case, the photons have sufficient energy to produce other particles when they reach the upper atmosphere, i.e., particles in the air, forming showers via cascading processes. This suggests the possibility of detecting gammas from the ground level, at least those that are more energetic and relatively long-term.

Despite great efforts for the systematic observation of GRBs at ground level, the results have been negative. However, a few events have been detected with a high significance, by the Tupi experiment in Brazil. They are considered as good candidates. Here, in addition to giving an overview of the GRBs, we will highlight these remarks (i.e., the Tupi events) and their implications on GRB physics.

#### **2. General features of GRBs**

Nowadays, the artificial production of pions (and consequently gamma rays) in N-N collisions is produced copiously. The artificial production of pions started in 1948, with Lattes and Gadner using the 184-inch synchrocyclotron at Lawrence Berkeley National Laboratory

As gamma rays coming from space are absorbed by the Earth's atmosphere, the first detection of gamma rays coming from space was through satellites. Indeed, the Explorer satellite in 1961 confirmed the existence of gamma rays in space [4]. Gamma rays coming from space have frequencies greater than about 1018 Hz; they occupy the same region of the electromagnetic spectrum as hard X-rays or above them. The only difference between them is their source: X-rays are produced by accelerating electrons, whereas gamma rays are produced by

There is a large variety of gamma-ray sources from space: the most important is the Sun's transient activity, such as solar flares and coronal mass ejection (CME). This means that there is also a "gamma-ray background" due to other stars in our galaxy and from the stars of other galaxies, as well as that expected from the interaction of cosmic rays (very energetically charged particles in space) with interstellar gas. There are also sources of gamma rays with continuous emission, or long-lived gamma-ray emission from compact sources. This emission is in the high-energy region from GeV to TeV. The Fermi Large Area Telescope (LAT)

The most intense are the galactic objects, such as the Crab nebula (Taurus constellation) and the W44 nebula (Aquila constellation), both two supernova remnants. There are also bright extragalactic objects, such as the objects called active galactic nuclei (AGN); the most bright are the so-called Markarian 501 and 421 AGNs. These extragalactic objects were discovered by a ground-based gamma-ray telescope, the "air Cherenkov" Wipple telescope [6], confirmed

Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe: they are bright flashes of gamma rays for a short period of time, in most cases less than 100 s, with a pho-

that a GRB of long duration (i.e., above 2 s). They are associated with exploding massive stars called as Hypernovas. The signature of these explosions is two opposing beams of gamma rays and if any one of these beams is in the Earth's direction, we will see the gamma-

In addition, sometimes, several narrow lines are detected; this radiation comes from the material around the explosion that was excited by the blast and that permits the detection of its host galaxy. However, sometimes, no narrow gamma-ray lines are detected, especially in GRBs of short duration (less than 2 s), meaning that some of these bursts come from different progenitors, such as merging compact objects such as black holes or neutron stars, or perhaps from a massive star explosion, but without the formation of a

/s/keV at Earth. There is some evidence that indicates

produced an inventory of 1873 objects shining in gamma-ray light [5].

(California) which accelerated protons to 350 MeV [2, 3].

atomic nuclei decays and/or nuclear collisions.

70 New Insights on Gamma Rays

by telescopes mounted on satellites.

ton's flux of around 0.1–100 ph/cm<sup>2</sup>

ray burst.

supernova.

The first scientific paper on GRBs was published in 1973 [7]. This paper reported the observation of 73 GRBs, starting on 2 July 1967, based on data from US satellites (Vela project) designed to monitor Russian nuclear weapon tests in space.

However, this first work did not answer several fundamental questions on the origin and nature of GRBs; for instance, are they from the solar system, Galactic halo, or are they extragalactic objects?

The mystery persisted until 1998, when the first spatial distribution of GRBs came from BATSE gamma detector and the more energetic (MeV to GeV) by EGRET, both onboard the Compton Gamma-Ray Observatory (CGRO). It was one of the best space observatories for detecting gamma rays from 20 keV to 30 GeV in Earth's orbit from 1991–2000. The BATSE instrument detected gamma-ray burst, at a rate of one per day, with a total of approximately 2700 detections. An isotropic sky distribution of GRBs was reported by BATSE. This means an extragalactic origin for the GRBs.

These observations were confirmed by new observations made by the new generations of GRBs detectors onboard satellites, such as the Swift, which is a multiwavelength GRB detector [8] with a wide field of view being able detect more than 100 GRBs per year. Swift has the Burst Alert Telescope (BAT) which covers the 15–150 keV energy band, the X-ray telescope (XRT), and an ultraviolet and optical telescope (UVOT) to detect X-ray and UV optical afterglows.

Swift has several online (free access) catalogs of GRBs. **Figure 1** shows the location in the sky (equatorial coordinate system) of 1188 Swift GRBs, detected from 17 December 2004 to 25 May 2016.

Thus, Swift confirmed the isotropic distribution of GRBs.

However, only the discovery of the first X-ray afterglows in 1998 by the BeppoSax satellite [9] allowed the accurate positions and the identification of the γ-ray afterglow with 'normal' galaxies to be obtained. It was also possible to obtain the redshifts and consequently their distances, confirming that GRBs have a cosmological origin.

**Figure 1.** Location in the sky (equatorial coordinate system) of 1188 Swift GRBs, detected from 17 December 2004 to 25 May 2016.

It was also the BATSE gamma detector onboard the CGRO that showed that there are two different types of GRBs; that is, GRBs are separated into two classes: long-duration bursts, normally from 2 to 500 s, with an average duration of about 30 s; and short-duration bursts, ranging from a few ms to 2 s, with an average duration of only 0.3 s, following a bimodal distribution. They are not small and large versions of the same phenomenon; the two types of bursts have completely different sources. The origin of long-duration bursts is linked to supernovae explosions of massive stars (with masses 100 times greater than the solar mass), called hypernovae. Short-duration bursts are linked to the merging of two objects, such as two neutron stars, or a neutron star and a black hole, or two black holes.

This bimodal distribution was also confirmed by Swift as shown in **Figure 2** (top panel). The figure includes also a scatter plot (bottom panel), a correlation between the T90 duration of the GRB and the integrated time fluence. T90 is defined as the time interval over which 90% of the total background-subtracted counts are observed, with the interval starting when 5% of the total counts have been observed. In addition, the integrated time fluence is the energy deposited on the detector per unit area, during the T90 duration of the GRBs. If the distance of the source of the GRB is determined, this last quantity allows estimate the energy released during the explosion. The energy output of GRBs on some cases, if spherically radiated is above 1054 erg. This exceeds any reasonable source during such a short timescale, so the radiation is likely highly beamed.

galaxies to be obtained. It was also possible to obtain the redshifts and consequently their

It was also the BATSE gamma detector onboard the CGRO that showed that there are two different types of GRBs; that is, GRBs are separated into two classes: long-duration bursts, normally from 2 to 500 s, with an average duration of about 30 s; and short-duration bursts, ranging from a few ms to 2 s, with an average duration of only 0.3 s, following a bimodal distribution. They are not small and large versions of the same phenomenon; the two types of bursts have completely different sources. The origin of long-duration bursts is linked to supernovae explosions of massive stars (with masses 100 times greater than the solar mass), called hypernovae. Short-duration bursts are linked to the merging of two objects, such as two

**Figure 1.** Location in the sky (equatorial coordinate system) of 1188 Swift GRBs, detected from 17 December 2004 to 25

This bimodal distribution was also confirmed by Swift as shown in **Figure 2** (top panel). The figure includes also a scatter plot (bottom panel), a correlation between the T90 duration of the GRB and the integrated time fluence. T90 is defined as the time interval over which 90% of the total background-subtracted counts are observed, with the interval starting when 5% of the total counts have been observed. In addition, the integrated time fluence is the energy deposited on the detector per unit area, during the T90 duration of the GRBs. If the distance of the source of the GRB is determined, this last quantity allows estimate the energy released during the explosion. The energy output of GRBs on some cases, if spherically radiated is above 1054 erg. This exceeds any reasonable source during such a short timescale, so the

neutron stars, or a neutron star and a black hole, or two black holes.

radiation is likely highly beamed.

May 2016.

72 New Insights on Gamma Rays

distances, confirming that GRBs have a cosmological origin.

**Figure 2.** Top panel: bimodal classification of Swift GRBs, as short and long. Bottom panel: correlation between the T90 duration of the GRBs and the integrated time fluence, on the basis of 1188 Swift GRBs, detected from 17 December 2004 to 25 May 2016.

#### **3. Emission mechanisms**

Despite remarkable progress in the past few years by theory and breakthroughs of observations, our understanding of these fascinating cosmic events is still very incomplete. Many aspects remain uncertain and demand further exploration. For instance, the detailed physics of the central engine is poorly understood. Even so, some GRB models can reproduce the main features of the observed bursts, irrespective of the detailed physics of the central engine.

The relativistic fireball GRB model was introduced in the 1990s [10, 11] on the basis of earlier works [12–14]. Although the model does not explain the central engine of a GRB, it has been successful in explaining the various features of GRBs, such as the origin of their afterglows.

According to this model, GRBs are produced far from the source (1011–1012 m), through the interactions between the outflow (fireball) and the surrounding medium (internal shocks). Following **Figure 3** that illustrates the generation mechanism of the GRBs, we can see that the X-rays afterglow result from the subsequent interaction of the outflow (fireball) with the surrounding medium (external shocks). The outflow fireball ends up losing its kinetic energy through successive interactions with the external medium, resulting in UV, visible, and radio afterglows.

**Figure 3.** Fireball model scheme of the generation mechanism of the GRBs. The GRBs are generated far from the source, through the interactions between the outflow (fireball) and the surrounding medium (internal shocks). The X-ray, UV, visible, and radio afterglow result from the subsequent interaction of the outflow (fireball) with the surrounding medium (external shocks).

The time variability of the gamma rays is 10−3 s, meaning a size of the emitting region of around 105 m; that is, a relativistic fireball, with a Lorentz factor above 1000 (Γ > 1000). The typical energy emitted in a collimated beaming flux is around 1045 J. A high Lorentz factor Γ also allows a relativistic collimated jet, with an aperture of θ ~1/Γ. As a consequence of this behavior, the relative angle at which photons collide decreases and leads to an increase in the pair production threshold.

**3. Emission mechanisms**

74 New Insights on Gamma Rays

medium (external shocks).

Despite remarkable progress in the past few years by theory and breakthroughs of observations, our understanding of these fascinating cosmic events is still very incomplete. Many aspects remain uncertain and demand further exploration. For instance, the detailed physics of the central engine is poorly understood. Even so, some GRB models can reproduce the main features of the observed bursts, irrespective of the detailed physics of the central engine. The relativistic fireball GRB model was introduced in the 1990s [10, 11] on the basis of earlier works [12–14]. Although the model does not explain the central engine of a GRB, it has been successful in explaining the various features of GRBs, such as the origin of their afterglows. According to this model, GRBs are produced far from the source (1011–1012 m), through the interactions between the outflow (fireball) and the surrounding medium (internal shocks). Following **Figure 3** that illustrates the generation mechanism of the GRBs, we can see that the X-rays afterglow result from the subsequent interaction of the outflow (fireball) with the surrounding medium (external shocks). The outflow fireball ends up losing its kinetic energy through successive interactions with the external medium, resulting in UV, visible, and radio afterglows.

The time variability of the gamma rays is 10−3 s, meaning a size of the emitting region of around 105 m; that is, a relativistic fireball, with a Lorentz factor above 1000 (Γ > 1000). The typical energy emitted in a collimated beaming flux is around 1045 J. A high Lorentz factor Γ also allows a relativistic collimated jet, with an aperture of θ ~1/Γ. As a consequence of this

**Figure 3.** Fireball model scheme of the generation mechanism of the GRBs. The GRBs are generated far from the source, through the interactions between the outflow (fireball) and the surrounding medium (internal shocks). The X-ray, UV, visible, and radio afterglow result from the subsequent interaction of the outflow (fireball) with the surrounding However, the simple relativistic fireball model produces a modified blackbody spectrum. This mechanism converts energy into thermal energy efficiently, thus it is necessary to reconvert kinetic energy into nonthermal emission, which happens when the fireball becomes optically thin. Thus, the reconverted kinetic energy into random energy must be via shocks, after the flow becomes optically thin (mainly synchrotron radiation).

In short, the fireball model can reproduce the main features of the observed bursts, irrespective of the detailed physics of the central engine.

Many GRB afterglow models [15–17] predict the production of photons in the GeV to TeV energy range, and GeV emission has indeed been detected by previous [18] and current-generation (Fermi LAT) space-based ray detectors [19].

The several GRBs observed by EGRET and Fermi LAT in the GeV energy region, as well as the ground-based observations of GRB candidates by Tupi suggest that the energy spectrum extend beyond GeV energies. In addition, there are some mechanism to explain this extension to very high energies, such as the synchrotron selfCompton model [20, 21], which provides a natural explanation for the optical and gamma-ray correlation seen in some GRBs. It has also been shown that a relatively strong second-order inverse Compton (IC) component of the GRB spectrum should peak in the 10s GeV energy region [22].

Another proposed model is the cannonball (CB). The cannonball model is inspired by observations of quasars and microquasars [23–25] and assumes that a supermassive star, when entering the final phase of its life, undergoes gravitational collapse, becoming a supernova (SN). In this internship, an accretion toroidal disk is developed around a compact object that is newly formed. The matter is then ejected as a CB in bipolar jets of plasma droplets (plasmoids) that are highly relativistic.

These jets collide with the photons inside the star through an inverse Compton scattering process, producing gamma rays. In this model, each pulse of GRB produced during the collapsing star corresponds to a CB.

The range of emission of these gamma rays is related to the layers (shields) of the star's interior, where these missiles collide. The CBs are individually ejected and the light curve observed in GRB depends on the local emission properties. As with the fireball model, the CB model can also describe the afterglows, such as X-ray, UV, and radio flares. The CB model also includes a description of other phenomena, such as the acceleration of cosmic rays (CRs) in a successful way [26].

The observed energy spectra of gamma-ray bursts reveal a diverse phenomenology. The spacecraft observed gamma-rays up to 33 GeV [27]. While some energy spectra have been fitted by a simple expression over many decades [28], others require a few separate components to explain high-energy emission [27]. In most cases (at low energies), the GRB spectrum is well described by a phenomenological "band function" in a "Comptonized model" using a power law with an exponential cutoff:

$$N(E) = K \, E^{-\alpha} \, e^{-\mathbb{E}|\mathbb{E}\_s|} \tag{1}$$

where *α* is the power-law exponent and *E*<sup>0</sup> is the cutoff energy. At high energies, the spectrum is described well by a power-law function with a steeper slope:

$$N(E) = A\_\gamma E^{\neg \beta} \tag{2}$$

where *α* > *β*, and the spectral parameters *α*, *β*, and *E*<sup>0</sup> vary from burst to burst. For instance, a "blast wave model," usually considered for GRB sources, is quite sensitive to the relationship between these two power-law indices.

#### **4. Ground level observations**

We present in this section a brief description of the various efforts for detecting at ground level, the GeV to TeV counterpart of the GRBs.

#### **4.1. Milagrito**

Milagrito was a detector (air shower detector, the predecessor of the Milagro detector) sensitive to very high-energy gamma rays, which monitored the northern sky with a large field of view and a high duty cycle, located near the Los Alamos Laboratory (New Mexico, USA). This instrument was well suited to perform a search for TeV gamma-ray bursts (GRBs). From February 1997 through May 1998, BATSE (Transient Satellite experiment) aboard the Compton Gamma-Ray Observatory detected 54 GRBs within the field of view of Milagrito.

The Milagrito results were negative; that is, no significant correlations were detected from the other bursts, with the exception of a single event, which was reported as evidence of a marginal emission at TeV energies from GRB 970417a. The event had a chance probability of 2.8 × 10−5 of being background fluctuation. The probability of observing an excess at least this large from any of the 54 bursts is 1.5 × 10−3 [29].

#### **4.2. Milagro**

Milagro was a wide-field (2 sr) high-duty cycle (>90%) ground-based water Cherenkov detector (60 m wide × 80 m long × 8 m deep) located at 2630 m above sea level in the Jemez mountains, New Mexico. Milagro had 723 PMTs divided into two layers under water. It triggers mainly on extensive air showers (EAS) in the energy range of 100 GeV to 100 TeV.

Milagro operated from January 2000 to May 2008. The gamma-ray coordinates network (GCN) system incorporated the distribution of positions of GRBs and transients detected by the MILAGRO instrument. However, none of these events was confirmed as true GRBs. Even so, Milagro succeeded in the detection of gamma rays in the TeV energy region, such as TeV gamma rays from the galactic plane [30], and the discovery of TeV gamma ray emission from the Cygnus region of the Galaxy [31]. Perhaps, its high energy threshold (above 100 GeV) set for gamma rays did not allow the detection of GRBs; thus only the upper limits were reported [32].

#### **4.3. ARGO**

*N*(*E*) = *K E*<sup>−</sup>*<sup>α</sup> e* <sup>−</sup>*E*/*E*<sup>0</sup> (1)

is the cutoff energy. At high energies, the spectrum

*N*(*E*) = *A<sup>γ</sup> E*<sup>−</sup>*<sup>β</sup>* (2)

vary from burst to burst. For instance, a

where *α* is the power-law exponent and *E*<sup>0</sup>

76 New Insights on Gamma Rays

between these two power-law indices.

**4. Ground level observations**

**4.1. Milagrito**

**4.2. Milagro**

level, the GeV to TeV counterpart of the GRBs.

large from any of the 54 bursts is 1.5 × 10−3 [29].

is described well by a power-law function with a steeper slope:

"blast wave model," usually considered for GRB sources, is quite sensitive to the relationship

We present in this section a brief description of the various efforts for detecting at ground

Milagrito was a detector (air shower detector, the predecessor of the Milagro detector) sensitive to very high-energy gamma rays, which monitored the northern sky with a large field of view and a high duty cycle, located near the Los Alamos Laboratory (New Mexico, USA). This instrument was well suited to perform a search for TeV gamma-ray bursts (GRBs). From February 1997 through May 1998, BATSE (Transient Satellite experiment) aboard the Compton Gamma-Ray Observatory detected 54 GRBs within the field of view of Milagrito. The Milagrito results were negative; that is, no significant correlations were detected from the other bursts, with the exception of a single event, which was reported as evidence of a marginal emission at TeV energies from GRB 970417a. The event had a chance probability of 2.8 × 10−5 of being background fluctuation. The probability of observing an excess at least this

Milagro was a wide-field (2 sr) high-duty cycle (>90%) ground-based water Cherenkov detector (60 m wide × 80 m long × 8 m deep) located at 2630 m above sea level in the Jemez mountains, New Mexico. Milagro had 723 PMTs divided into two layers under water. It triggers

Milagro operated from January 2000 to May 2008. The gamma-ray coordinates network (GCN) system incorporated the distribution of positions of GRBs and transients detected by the MILAGRO instrument. However, none of these events was confirmed as true GRBs. Even so, Milagro succeeded in the detection of gamma rays in the TeV energy region, such as TeV gamma rays from the galactic plane [30], and the discovery of TeV gamma ray emission from the Cygnus region of the Galaxy [31]. Perhaps, its high energy threshold (above 100 GeV) set for gamma rays did not allow the detection of GRBs; thus only the upper limits were reported [32].

mainly on extensive air showers (EAS) in the energy range of 100 GeV to 100 TeV.

where *α* > *β*, and the spectral parameters *α*, *β*, and *E*<sup>0</sup>

The Astrophysical Radiation with Ground-based Observatory at Yangbajing, China (Tibet— 4300 m a.s.l.), under the auspices of the ARGO-YBJ experiment, is through an air shower detector. The ARGO-YBJ detector has a large active surface of around 6700 m2 of Resistive Plate Chambers, a wide field of view ~2 sr, and a high duty cycle (>86%). The ARGO-YBJ experiment is a collaboration of Italian and Chinese institutions [33].

The ARGO-YBJ performed a search for gamma-ray bursts (GRB) emission in the energy range 1–100 GeV in coincidence with satellite detection. From 17 December 2004 to 7 February 2013, a total of 206 GRBs occurring within the ARGO-YBJ field of view (zenith angle θ = 45°) were analyzed, no significant excess was found, and only the corresponding fluence upper limits in the 1–100 GeV energy region were derived, with values as low as 10−5 erg/cm2 .

#### **4.4. HAWC**

The HAWC gamma-ray observatory is a wide field of view, continuously operating, TeV gamma-ray telescope that explores the origin and solar modulation of cosmic rays and searches for new TeV physics. HAWC is located at a high altitude of 4100 m above sea level in Mexico (Sierra Negra) and is a collaboration of 15 US and 12 Mexican institutions.

HAWC consists of an array of 300 water Cherenkov detectors and is expected to be more than one order of magnitude, which is more sensitive than its predecessor, Milagro. HAWC monitors the northern sky and makes coincident observations with other wide field of view observatories. The HAWC experiment is particularly suitable to detect short and unexpected events like GRBs. However, thus far, no excess candidate events, nor GRB counterparts, have been reported.

Many other experiments have searched for the GeV-TeV counterpart of GRBs. No conclusive detection such as INCA [34], Tibet AS [35], HEGRA AIROBICC [36], GRAND [37], LAGO [38], and the Cherenkov detector MAGIC have given a very low upper limit between 85 and 1000 GeV [39]. In short, no significant correlations among events of these experiments and Satellite GRBs observations have been related.

#### **5. Ground level observation of gamma-ray bursts from space**

The Earth's magnetic field effects on the development of a particle shower in the atmosphere spread the collecting particles and therefore decrease the sensitivity of the detector. This deflection is caused by the component of the Earth's magnetic field perpendicular to the particle trajectory. Thus, if the source of gamma rays is close to the vertical direction, the places with the smaller horizontal magnetic component will be the best places, to ground level detection of GRBs.

We point out that the location of the Tupi detector is within the South Atlantic Anomaly (SAA). It is the region characterized by anomalously weak geomagnetic field strength. It is the lowest magnetic field of the world. The SAA central region is located on 26° S, 53° W, over the South Atlantic Ocean on Brazil, close to the Tupi detector location. The horizontal geomagnetic field component is only 18.13 mT, that is, almost half than the horizontal geomagnetic field component of other locations, where a search for the detection of GRBs at ground level was performed. For details, please see Section 7 of the reference [40].

Since August 2013, the Tupi experiment has operated an extended array of five muon telescopes [40], located in Niteroi, Rio de Janeiro, Brazil, (22.9° S, 43.0° W).

The first has a vertical orientation. The other four have orientations to the north, south, east, and west; each telescope is inclined 45° relative to the vertical.

Each telescope was constructed on the basis of two detectors (plastic scintillators 50 × 50 × 3 cm) separated by a distance of 3 m. The coincident signals in the upper and lower Tupi detectors are registered at a rate of 1 Hz.

There are two flagstones of concrete (150 g cm−2) above telescopes and only particles (muons) with energies above 0.1–0.2 GeV can penetrate the two flagstones. This defines the energy threshold of the telescopes. Each Tupi telescope has an effective field of view of 0.37 sr. For the vertical telescope, this corresponds to an aperture (zenith angle) of 20°.

We present ground level observations in the GeV energy range of possible counterparts associated with the gamma-ray bursts observed by spacecraft detectors [41], such as the MAXI onboard the ISS and the BAT onboard Swift [42].

In the period from 8 September 2013 to 10 August 2014, 34 GRBs observed by satellites in the keV energy region were within the field of view of the five Tupi telescopes. The majority of the events was compatible with the Tupi background fluctuations. The exceptions were the two events that are described below.

In addition, of the 34 GRBs with energies above 100 MeV observed by the Femi LAT detector up until 10 August 2014 [43], only one GRB131018B had their trigger coordinates within the field of view of one Tupi telescope. However, no signal was found. **Figure 4** shows the location in equatorial coordinates of the four Tupi telescopes and includes the trigger coordinates of the Fermi LAT GRBs until 10 August 2014.

#### **5.1. Association with the MAXI gamma-ray burst**

On 15 October 2013 at 21:55:44 UT, a peak (muon excess) with a significance of 5 s at the 68% confidence level was found in the 24 h raw data (counting rate 1 Hz) of the vertical Tupi telescope.

It was possible to recognize this peak in the time profile of the muon counting rate, just by the naked eye, as shown in **Figure 5**. The peak was found at To + 25.7 s, where To = 21:55:19 UT is the occurrence of the MAXI trigger [44]. The trigger coordinates of MAXI trigger were within the field of view of the vertical Tupi telescope.

In addition, a second narrow peak with a significance of 4 sigma (1 s binning) can be observed at To + 297.2 s. This peak can be seen also at the 3 and 5 s binning counting rates, as shown in **Figure 5**. This behavior strongly suggests that this peak is a true signal.

In order to see the background fluctuations more accurately, we examined the time profiles up to 30 min before and after the trigger time. A confidence analysis was made for a 1 h interval around the MAXI trigger time, as shown in **Figure 6**. In the absence of a signal, the background fluctuation of the counting rate follows a Gaussian distribution. Thus, the trials out of the Gaussian curve are considered as a signal's signature.

field component is only 18.13 mT, that is, almost half than the horizontal geomagnetic field component of other locations, where a search for the detection of GRBs at ground level was

Since August 2013, the Tupi experiment has operated an extended array of five muon tele-

The first has a vertical orientation. The other four have orientations to the north, south, east,

Each telescope was constructed on the basis of two detectors (plastic scintillators 50 × 50 × 3 cm) separated by a distance of 3 m. The coincident signals in the upper and lower Tupi detectors

There are two flagstones of concrete (150 g cm−2) above telescopes and only particles (muons) with energies above 0.1–0.2 GeV can penetrate the two flagstones. This defines the energy threshold of the telescopes. Each Tupi telescope has an effective field of view of 0.37 sr. For the

We present ground level observations in the GeV energy range of possible counterparts associated with the gamma-ray bursts observed by spacecraft detectors [41], such as the MAXI

In the period from 8 September 2013 to 10 August 2014, 34 GRBs observed by satellites in the keV energy region were within the field of view of the five Tupi telescopes. The majority of the events was compatible with the Tupi background fluctuations. The exceptions were the

In addition, of the 34 GRBs with energies above 100 MeV observed by the Femi LAT detector up until 10 August 2014 [43], only one GRB131018B had their trigger coordinates within the field of view of one Tupi telescope. However, no signal was found. **Figure 4** shows the location in equatorial coordinates of the four Tupi telescopes and includes the trigger coordinates

On 15 October 2013 at 21:55:44 UT, a peak (muon excess) with a significance of 5 s at the 68% confidence level was found in the 24 h raw data (counting rate 1 Hz) of the vertical Tupi telescope. It was possible to recognize this peak in the time profile of the muon counting rate, just by the naked eye, as shown in **Figure 5**. The peak was found at To + 25.7 s, where To = 21:55:19 UT is the occurrence of the MAXI trigger [44]. The trigger coordinates of MAXI trigger were within

In addition, a second narrow peak with a significance of 4 sigma (1 s binning) can be observed at To + 297.2 s. This peak can be seen also at the 3 and 5 s binning counting rates, as shown in

In order to see the background fluctuations more accurately, we examined the time profiles up to 30 min before and after the trigger time. A confidence analysis was made for a 1 h

**Figure 5**. This behavior strongly suggests that this peak is a true signal.

performed. For details, please see Section 7 of the reference [40].

and west; each telescope is inclined 45° relative to the vertical.

are registered at a rate of 1 Hz.

78 New Insights on Gamma Rays

scopes [40], located in Niteroi, Rio de Janeiro, Brazil, (22.9° S, 43.0° W).

vertical telescope, this corresponds to an aperture (zenith angle) of 20°.

onboard the ISS and the BAT onboard Swift [42].

two events that are described below.

of the Fermi LAT GRBs until 10 August 2014.

the field of view of the vertical Tupi telescope.

**5.1. Association with the MAXI gamma-ray burst**

**Figure 4.** Equatorial coordinates showing the positions of the five Tupi telescope axes, as well as the Fermi LAT GRBs (>100 MeV) in the period until 10 August 2014. The squares with circles represent the FOV of the Tupi telescopes, which were within the FOV of the North Tupi telescope.

**Figure 5.** Statistical significance (i.e., number of standard deviations) of the 1, 3, and 5 s binning counting rates observed by the vertical Tupi telescope, as a function of the time elapsed since the MAXI transient 580727270 trigger time.

**Figure 6.** Distribution of the fluctuation count rate for the Tupi telescope (in units of standard deviations) within a temporal windows of 30 min around the MAXI transient event (trigger 580727270). The solid curve represents a Gaussian distribution (background fluctuation) and the signals associated with the MAXI events with significance above 4 sigma clearly are outside of the Gaussian distribution.

We estimated the Poisson probability of the counting rate excess observed in the vertical Tupi telescope, in association with the MAXI GRB events, being a background fluctuation, as P = (1.6 ± 0.2) × 10−9, i.e., an annual rate of 2.9.

In addition, from spectral analysis, the fluence of the first peak (at To + 25.7 s) was estimated as F = (2.1 ± 0.4) × 10−7 erg/cm2 .

#### **5.2. Association with the Swift gamma-ray bursts**

According to Pagani et al. (GCN 16249), on 12 May 2014 at 19:31:49 UT, the Swift BAT triggered and located a multipeak event with a total duration of about 170 s cataloged as GRB140512A (trigger = 598819). This Swift event is also in temporal association with the Fermi gamma-ray burst monitor event (Stanbro, GCN 16262). The calculated location by Swift-BAT was (R.A., decl.) = (289.371, −15.100).

**Figure 7.** Top panels: the counting rate of gamma rays for five energy ranges for the event Swift BAT GRB140512A. Bottom panel: time profile of the counting rate 4 s binning, and expressed as the number of standard deviations, observed in the Tupi vertical telescope as a function of the time elapsed since the Swift BAT GRB140512A trigger time.

We estimated the Poisson probability of the counting rate excess observed in the vertical Tupi telescope, in association with the MAXI GRB events, being a background fluctuation, as

**Figure 6.** Distribution of the fluctuation count rate for the Tupi telescope (in units of standard deviations) within a temporal windows of 30 min around the MAXI transient event (trigger 580727270). The solid curve represents a Gaussian distribution (background fluctuation) and the signals associated with the MAXI events with significance above 4 sigma

In addition, from spectral analysis, the fluence of the first peak (at To + 25.7 s) was estimated

According to Pagani et al. (GCN 16249), on 12 May 2014 at 19:31:49 UT, the Swift BAT triggered and located a multipeak event with a total duration of about 170 s cataloged as GRB140512A

P = (1.6 ± 0.2) × 10−9, i.e., an annual rate of 2.9.

clearly are outside of the Gaussian distribution.

80 New Insights on Gamma Rays

**5.2. Association with the Swift gamma-ray bursts**

.

as F = (2.1 ± 0.4) × 10−7 erg/cm2

An excess in the Tupi counting rate with a significance of 4.55 sigma was found, temporally and spatially associated with the Swift BAT GRB140512A. The trigger coordinates of this event were very close to the zenith of the Tupi location, that is, within the FOV of the vertical telescope. The signal at Tupi is within the T90 duration of the Swift GRB140512A. **Figure 7** summarizes the situation, where a comparison between the time profiles of Swift BAT and Tupi is shown, as a function of the time elapsed since the Swift BAT GRB140512A trigger time.

The peak in the time profiles of Tupi associated with the Swift GRB140512A persist with the same confidence in the 1, 3, 5, and 10 s binning, as shown in **Figure 8**. This means that the peak is not subjected to be the only background fluctuation.

**Figure 8.** Time profiles observed by the Tupi vertical telescope and expressed as the statistical significance (i.e., number of standard deviations) as a function of the time, since the Swift GRB140512A trigger time and for 1, 3, 5, and 10 s binning. The yellow band marks the region surrounding the Swift trigger time.

To see the expected background fluctuations, a confidence analysis was performed for a 1 h interval around the Swift BAT trigger time, as shown in **Figure 9**. The excess above the Gaussian curve (at right) is linked to the Tupi telescope's signal, associated with the Swift GRB event.

We also estimated the Poisson probability of the excess observed in the counting rate in the vertical Tupi telescope, in association with the Swift GRB event, being a background fluctuation, as P = (7.40 ± 1.21) × 10−6, i.e., an annual rate of 73.1.

**Figure 9.** Distribution of the fluctuation count rate for the Tupi telescope (in units of standard deviations) within a temporal windows of 30 min around the Swift GRB140512A transient event. The solid curve represents a Gaussian distribution (background fluctuation) and the signals associated with the Swift events, with significance above 4 sigma clearly are outside of the Gaussian distribution.

#### **6. Summary**

this event were very close to the zenith of the Tupi location, that is, within the FOV of the vertical telescope. The signal at Tupi is within the T90 duration of the Swift GRB140512A. **Figure 7** summarizes the situation, where a comparison between the time profiles of Swift BAT and Tupi is shown, as a function of the time elapsed since the Swift BAT GRB140512A

The peak in the time profiles of Tupi associated with the Swift GRB140512A persist with the same confidence in the 1, 3, 5, and 10 s binning, as shown in **Figure 8**. This means that the peak

To see the expected background fluctuations, a confidence analysis was performed for a 1 h interval around the Swift BAT trigger time, as shown in **Figure 9**. The excess above the Gaussian curve (at right) is linked to the Tupi telescope's signal, associated with the Swift

**Figure 8.** Time profiles observed by the Tupi vertical telescope and expressed as the statistical significance (i.e., number of standard deviations) as a function of the time, since the Swift GRB140512A trigger time and for 1, 3, 5, and 10 s

We also estimated the Poisson probability of the excess observed in the counting rate in the vertical Tupi telescope, in association with the Swift GRB event, being a background fluctua-

tion, as P = (7.40 ± 1.21) × 10−6, i.e., an annual rate of 73.1.

binning. The yellow band marks the region surrounding the Swift trigger time.

is not subjected to be the only background fluctuation.

trigger time.

82 New Insights on Gamma Rays

GRB event.

We have carried out a systematic search for a GeV counterpart observed at ground level of GRBs triggered in gamma-ray detectors onboard of satellite. An overview on the gamma rays from space was given and the main features observed in detectors onboard satellites, since their discovery in the 1960s and their most recent observations. A brief report is also presented on the main possible mechanisms, as the fireball and the cannonballs models. Both can reproduce the main features of the observed bursts, irrespective of the detailed physics of the central engine.

In addition, several scenarios have been indicated to explain a possible high-energy component of GRBs, such as the synchrotron selfCompton model and the second-order inverse Compton component of the GRB spectrum.

We also included a chronological description of the various efforts for detecting at ground level, the high energy component, that is, the GeV to TeV counterpart of the GRBs. In most cases, ground level detectors have an high energy detection threshold of the secondary particles detected (above 100 GeV); it has not allowed the detection at ground level.

We highlight that the location of the Tupi detector is within the South Atlantic Anomaly (SAA); it allows to achieve higher sensitivity, and some candidates to the GeV counterpart of gamma ray bursts, observed by Tupi telescopes were presented. They are in correlation with temporal and spatial GRBs detected by satellites.

Of course, that the Tupi detector has recorded excess in the counting rate, in correlation with the gamma-ray bursts. As the detector is located at sea level, it is expected that this excess is principally produced by muons. However, the assumption of photomuons as the origin of the excess requires gamma rays with energies above 10 GeV.

In addition, there is an alternative mechanism that is useful to explain high-energy electrons from terrestrial gamma-ray flashes [45] and observations of gamma-ray bursts at ground level under thunderclouds [46]. However, this mechanism requires some special conditions, such as an atmospheric high electric field.

The mechanism is known as "Relativistic runaway electron avalanche in the atmosphere". An initial energetic electron is needed to start the process. In the atmosphere, such energetic electrons typically come from cosmic rays; for instance, gamma rays via pair production process *γ* → *e*<sup>+</sup> + *e*<sup>−</sup> in the upper atmosphere. In this case, there are several seeds for the generation of the successive avalanches; if the atmospheric electric field region is large enough, the number of second-generation avalanches (i.e., avalanches produced by avalanches) will exceed the number of first-generation avalanches, and the number of avalanches itself grows exponentially. This avalanche of avalanches can produce extremely large populations of energetic electrons [47].

Clearly, more studies are needed in order to establish whether this mechanism has the potential to explain the excesses observed in the counting rate of the Tupi detector associated with GRBs. So far, the mechanism "Relativistic runaway electron avalanche in the atmosphere" is the only promising one.

The implications of these ground level observations show that GRBs of long duration have chances of having a GeV counterpart; this characteristic can be useful to the formulations of possible mechanisms of a GeV emission. The experiment is in progress, and the aim is to obtain a large number of candidates in the next years to obtain some systemic features of this phenomenon.

#### **Acknowledgements**

The support from National Council for Research (CNPq) and Fundacao de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) both in Brazil is gratefully acknowledged. We also express our gratitude to V. Kopenkin, C. R. A. Augusto, and A. Nepomuceno for their help in the analysis and to the Goddard Space Flight Center (NASA) for the free access to data through GCN web page (http://gcn.gsfc.nasa.gov/).

#### **Author details**

cases, ground level detectors have an high energy detection threshold of the secondary par-

We highlight that the location of the Tupi detector is within the South Atlantic Anomaly (SAA); it allows to achieve higher sensitivity, and some candidates to the GeV counterpart of gamma ray bursts, observed by Tupi telescopes were presented. They are in correlation with

Of course, that the Tupi detector has recorded excess in the counting rate, in correlation with the gamma-ray bursts. As the detector is located at sea level, it is expected that this excess is principally produced by muons. However, the assumption of photomuons as the origin of the

In addition, there is an alternative mechanism that is useful to explain high-energy electrons from terrestrial gamma-ray flashes [45] and observations of gamma-ray bursts at ground level under thunderclouds [46]. However, this mechanism requires some special conditions, such

The mechanism is known as "Relativistic runaway electron avalanche in the atmosphere". An initial energetic electron is needed to start the process. In the atmosphere, such energetic electrons typically come from cosmic rays; for instance, gamma rays via pair production

eration of the successive avalanches; if the atmospheric electric field region is large enough, the number of second-generation avalanches (i.e., avalanches produced by avalanches) will exceed the number of first-generation avalanches, and the number of avalanches itself grows exponentially. This avalanche of avalanches can produce extremely large populations of

Clearly, more studies are needed in order to establish whether this mechanism has the potential to explain the excesses observed in the counting rate of the Tupi detector associated with GRBs. So far, the mechanism "Relativistic runaway electron avalanche in the atmosphere" is

The implications of these ground level observations show that GRBs of long duration have chances of having a GeV counterpart; this characteristic can be useful to the formulations of possible mechanisms of a GeV emission. The experiment is in progress, and the aim is to obtain a large number of candidates in the next years to obtain some systemic features of this phenomenon.

The support from National Council for Research (CNPq) and Fundacao de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) both in Brazil is gratefully acknowledged. We also express our gratitude to V. Kopenkin, C. R. A. Augusto, and A. Nepomuceno for their help in the analysis and to the Goddard Space Flight Center (NASA) for the free access to data

in the upper atmosphere. In this case, there are several seeds for the gen-

ticles detected (above 100 GeV); it has not allowed the detection at ground level.

temporal and spatial GRBs detected by satellites.

as an atmospheric high electric field.

process *γ* → *e*<sup>+</sup> + *e*<sup>−</sup>

84 New Insights on Gamma Rays

energetic electrons [47].

the only promising one.

**Acknowledgements**

through GCN web page (http://gcn.gsfc.nasa.gov/).

excess requires gamma rays with energies above 10 GeV.

Carlos Navia\* and Marcel Nogueira de Oliveira

\*Address all correspondence to: tupi.carlos24@gmail.com

Physical Institute, Universidade Federal Fluminense, Niterói, Brazil

#### **References**


[26] De Rújula A. A unified model of high-energy astrophysical phenomena. International Journal of Modern Physics A. 2005;**20**(29):6562–6583. DOI: 10.1142/S0217751X05029617

[12] Cavallo G., Rees M. J. A qualitative study of cosmic fireballs and γ-ray bursts. Monthly Notices of the Royal Astronomical Society. 1978;**183**(3):359–365. DOI: 10.1093/

[13] Paczynski B. Gamma-ray bursters at cosmological distances. The Astrophysical Journal.

[14] Shemi A., Piran T. The appearance of cosmic fireballs. The Astrophysical Journal.

[15] Wang X. Y., Dai Z. G., Lu T. The inverse Compton emission spectra in the very early afterglows of gamma-ray bursts. The Astrophysical Journal. 2001;**556**(2):1010. DOI:

[16] Zhang B., Meszaros P. High-energy spectral components in gamma-ray burst afterglows. The Astrophysical Journal. 2001;**559**(1):110–122. DOI: 10.1086/322400

[17] Pe'er A., Waxman E. The high-energy tail of GRB 941017: comptonization of synchrotron self-absorbed photons. The Astrophysical Journal Letters. 2004;**603**(1):L1–L4. DOI:

[18] Sommer M., Bertsch D. L., Dingus B. L., Fichtel C. E., Fishman G. J., Harding A. K., et al. High-energy gamma rays from the intense 1993 January 31 gamma-ray burst. The

[19] Abdo A. A., Ackermann M., Ajello M., Atwood W. B., Axelsson M., Baldini L., et al. Fermi/large area telescope bright gamma-ray source list. The Astrophysical Journal

[20] Spada M., Panaitescu A., Meszaros P. Analysis of temporal features of gamma-ray bursts in the internal shock model. The Astrophysical Journal. 2000;**537**(2):824–832. DOI:

[21] Kumar P., McMahon E. A general scheme for modelling γ-ray burst prompt emission. Monthly Notices of the Royal Astronomical Society. 2008;**384**(1):33–63. DOI:

[22] Racusin J. L., Karpov S. V., Sokolowski M., Granot J., Wu X. F., Pal'Shin V., et al. Broadband observations of the naked-eye big gamma-ray burst GRB 080319B. Nature.

[23] Mirabel I. F., Rodriguez L. F. Sources of relativistic jets in the Galaxy. Annual Review of Astronomy and Astrophysics. 1999;**37**:409–443. DOI: 10.1146/annurev.astro.37.1.409

[24] Rodriguez L. F., Mirabel I. F. Repeated relativistic ejections in GRS 1915+ 105. The

[25] Dar A. Fireball and cannonball models of gamma-ray bursts confront observations. Chinese Journal of Astronomy and Astrophysics. 2006;**6**(S1):301–314. DOI:

Astrophysical Journal. 1999;**511**(1):398–404. DOI: 10.1086/306642

Astrophysical Journal. 1994;**442**:L63–L66. DOI: 10.1086/187213

Supplement Series. 2009;**183**(1):46–66. DOI: 10.1088/0067-0049/183/1/46

mnras/183.3.359

86 New Insights on Gamma Rays

10.1086/321608

10.1086/382872

10.1086/309048

10.1111/j.1365-2966.2007.12621.x

10.1088/1009-9271/6/S1/39

2008;**455**(7210):183–188. DOI: 10.1038/nature07270

1986;**308**:L43–L46. DOI: 10.1086/184740

1990;**365**:L55–L58. DOI: 10.1086/185887


### **Extragalactic Gamma‐Ray Background**

#### Houdun Zeng and Li Zhang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67335

#### Abstract

[38] Allard D., Allekotte I., Alvarez C., Asorey H., Barros H., Bertou X., et al. Use of water-Cherenkov detectors to detect gamma ray bursts at the large aperture GRB observatory (LAGO). Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 2008;**595**(1):70–72. DOI: 10.1016/j.

[39] Albert J., Aliu E., Anderhub H., Antoranz P., Armada A., Asensio M., et al. Flux upper limit on gamma-ray emission by GRB 050713a from magic telescope observations. The

[40] Augusto C. R. A., Navia C. E., Shigueoka H., Tsui K. H., Fauth A. C. Muon excess at sea level from solar flares in association with the Fermi GBM spacecraft detector. Physical

[41] NASA. GCN: The Gamma-ray Coordinates Network (TAN: Transient Astronomy Network) [Internet]. Available from: gcn.gsfc.nasa.gov [Accessed: July 19, 2016].

[42] Augusto C. R. A., Navia C. E., De Oliveira M. N., Tsui K. H., Nepomuceno A. A., Kopenkin V., et al. Observation of Muon excess at ground level in relation to gammaray bursts detected from space. The Astrophysical Journal. 2015;**805**(1):69. DOI:

[43] Ackermann M., Ajello M., Asano K., Axelsson M., Baldini L., Ballet J., et al. The first Fermi-LAT gamma-ray burst catalog. The Astrophysical Journal Supplement Series.

[44] Matsuoka M., Kawasaki K., Ueno S., Tomida H., Kohama M., Suzuki M., et al. The MAXI mission on the ISS: science and instruments for monitoring all-sky X-ray images. Publications of the Astronomical Society of Japan. 2009;**61**(5):999–1010. DOI: 10.1093/

[45] Dwyer J. R., Smith D. M. A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma-ray flash observations. Geophysical Research Letters.

[46] Kuroda Y., Oguri S., Kato Y., Nakata R., Inoue Y., Ito C., et al. Observation of gamma ray bursts at ground level under the thunderclouds. Physics Letters B. 2016;**758**:286–291.

[47] Dwyer J. R. A fundamental limit on electric fields in air. Geophysical Research Letters.

Astrophysical Journal Letters. 2006;**641**(1):L9–L12. DOI: 10.1086/503767

Review D. 2011;**84**(4):042002. DOI: 10.1103/PhysRevD.84.042002

nima.2008.07.041

88 New Insights on Gamma Rays

10.1088/0004-637X/805/1/69

pasj/61.5.999

2013;**209**(1):11. DOI: 10.1088/0067-0049/209/1/11

2005;**32**(22):L22804. DOI: 10.1029/2005GL023848

2003;**30**(20):2055. DOI: 10.1029/2003GL017781

DOI: 10.1016/j.physletb.2016.05.029

The origin of the extragalactic gamma-ray background (EGRB) is an important open issue in the gamma-ray astronomy. There are many theories about the origin of EGRB: (1) some truly diffuse processes, such as dark matter (DM) annihilation or decay, which can produce gamma rays; (2) gamma rays produced by energetic particles accelerated through induced shock waves during structure formation of the universe; (3) a lot of unidentified sources, including normal galaxies, starbursts and active galactic nuclei (AGNs), contain a large number of energetic particles and can emit gamma rays. Among various extragalactic sources, blazars including flat spectral radio quasars (FSRQs) and BL Lac objects are one of the most possible sources for EGRB. As continuous accumulation of the data observed by the Fermi Gamma-Ray Space Telescope, it is possible to directly construct gamma-ray luminosity function (GLF) of the blazars involving evolution information. In this chapter, based on the largest clean sample of AGNs provided by Fermi Large Area Telescope (LAT), we mainly study blazar's GLFs and their contribution to EGRB. In our study, we separately construct GLFs of FSRQs and BL Lacs and then estimate the contributions to EGRB, respectively. Further, we discuss the diffuse gamma ray from other astrophysical sources and the other possible origins of the EGRB.

Keywords: blazars, gamma-ray radiation, luminosity function, the extragalactic gamma-rays background

#### 1. Introduction

The large area telescope (LAT [1]) onboard Fermi gamma-ray space telescope (Fermi) has measured the extragalactic diffuse gamma-ray background and then provided useful information for us to study the origins of the extragalactic gamma-ray background (EGRB) [2–5]. However, the origin of the EGRB is still an unsolved problem. Observationally, an isotropic component of the EGRB emission was first detected by the SAS-2 satellite [6, 7] and subsequently measured by the energetic gamma-ray experiment telescope (EGRET) [8–10]. Due to the higher sensitivity of Fermi-LAT than that of EGRET, the observed integrated flux above 100MeV by the LAT is

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

<sup>ð</sup>1:03–0:17<sup>Þ</sup> · <sup>10</sup><sup>−</sup><sup>5</sup> photons cm<sup>−</sup>2s<sup>−</sup><sup>1</sup> [3], which is lower than <sup>ð</sup>1:14–0:05<sup>Þ</sup> · <sup>10</sup><sup>−</sup><sup>5</sup> photons cm<sup>−</sup>2s<sup>−</sup><sup>1</sup> measured by EGRET [11]. Recently, Fermi-LAT has made a new measurement of the EGRB spectrum and their results shown that the EGRB energy spectrum between 0.1 and 820GeV is to be well represented by a power law with an exponential cutoff above 300GeV [5]. Figure 1 (left panel) shows the measured X-ray and gamma-ray background radiation spectra. We know that the X-ray background spectrum has no big change with time and has been considered as the integrated light produced via the accretion process of active galactic nuclei (AGNs) [12]. However, the gamma-ray spectrum is different from the X-ray background spectrum due to the sensitivity of an instrument and other reasons. Before the Fermi gamma-ray space telescope era, neither spectrum nor origin of the EGRB was well understood. In particular, the spectrum at 0.03–50GeV reported by EGRET has a break in the several GeV. With the arrival of Fermi era, more accurate determination of the EGRB spectrum and more extragalactic source samples are provided to understand the nature of the EGRB. Note that the whole gamma-ray sky contains diffuse galactic emission, point sources, isotropic extragalactic diffuse emission and local and solar diffuse emissions. Figure 1 (right panel) shows that the EGRB spectrum is obtained by removing the resolved point source, like as the most recent list of resolved Fermi-LAT source (3FGL), the diffuse galactic emission determined by GALPROP, which simulates both cosmic-ray propagation in the galaxy and the gamma-ray flux resulting from interactions and possibly an isotropic flux of galactic, by restricting data to regions with |b|>10° or even higher galactic latitudes.

Figure 1. Left: The measured X-ray and gamma-ray background radiation spectra, which is from Ref. [5]. Right: The composition of the total gamma-ray flux. The figure is obtained from the report of Ackermann, M. at Fermi Symposium.

Similar to the extragalactic EGRET sky, blazars are the largest source class identified by Fermi extragalactic sky and their contribution to the EGRB has been widely discussed. Typical estimated contributions of unresolved blazars to the EGRB range from 10 to 100% [13–36]. Blazars are divided into two main subgroups: BL Lac objects and FSRQs [37]. Among the gamma-ray blazar sample, the number of FSRQs detected by Fermi-LAT is smaller than that of BL Lac objects (e.g., 2FGL, 3FGL). FSRQs generally show softer spectrum in the gamma-ray band (e.g., [38]), which is to be detected harder than BL Lac objects at a given significant limit. On the one hand, BL Lacs are reputed as the population of extragalactic sources that show a negative or no cosmological evolution [39–42], but FSRQs are regarded as those with a positive cosmological evolution, which

is similar to the population of X-ray-selected, radio-quiet AGNs [43–45]. Ajello et al. [32] suggested that BL Lacs have a more complex evolution. At the modest redshift region, most BL Lac classes show a positive evolution with a space density peaking. Meanwhile, their results suggest that the evolution of low-luminosity, high-synchrotron-peaked (HSP) BL Lac objects is strong negative with number density increasing for low redshift range (z ≤ 0:5). In addition, the contributions of the EGRB from other sources or processes are very important. Those are starforming galaxies [46, 47], radio galaxies (e.g., [14, 46, 48]), gamma-ray bursts (GRBs) (e.g., [49]), high galactic-latitude pulsars (e.g., [50]), intergalactic shocks (e.g., [51, 52]), Seyferts (e.g., [53]), cascade from ultra-high-energy cosmic rays (e.g., [54, 55]), large galactic electron halo [56], cosmicray interaction in the solar system [57] and dark matter annihilation or decay (e.g., [58]). Recently, with the assumptions and uncertainties, Ajello et al. [33] and Di Mauro and Donato [36] shown that the EGRB can be fully accounted for the sum of contributions from undetected sources including blazars and radio and star-forming galaxies. Those results imply that little room in space is left for other processes such as shock wave or DM interactions (e.g., [33, 59]).

The extragalactic gamma-ray sky provides an amount of gamma-ray sources and allows us to obtain the information about the evolution of sources and estimate their contributions to the EGRB. Because the blazar's contribution is the main content of research on this chapter, the detail about how to build the gamma-ray luminosity function (GLF) will be discussed in Section2. In Section3, a brief description about how to estimate different components' contributions to the EGRB is given and finally, we give the conclusions and discussions in Section4.

#### 2. The gamma-ray luminosity function

Since the Fermi-LAT has detected and identified more and more gamma-ray sources and observed previously detected objects in greater detail, the method by using the gamma-ray luminosity function (GLF) to estimate the EGRB of resolved sources has become much more reliable. In this approach, the GLF involving the evolution of redshift as well as the distribution of spectral indices of a given source class can be established for all known sources and the observed population can be extrapolated to lower fluxes.

#### 2.1. Function derivation

<sup>ð</sup>1:03–0:17<sup>Þ</sup> · <sup>10</sup><sup>−</sup><sup>5</sup> photons cm<sup>−</sup>2s<sup>−</sup><sup>1</sup> [3], which is lower than <sup>ð</sup>1:14–0:05<sup>Þ</sup> · <sup>10</sup><sup>−</sup><sup>5</sup> photons cm<sup>−</sup>2s<sup>−</sup><sup>1</sup> measured by EGRET [11]. Recently, Fermi-LAT has made a new measurement of the EGRB spectrum and their results shown that the EGRB energy spectrum between 0.1 and 820GeV is to be well represented by a power law with an exponential cutoff above 300GeV [5]. Figure 1 (left panel) shows the measured X-ray and gamma-ray background radiation spectra. We know that the X-ray background spectrum has no big change with time and has been considered as the integrated light produced via the accretion process of active galactic nuclei (AGNs) [12]. However, the gamma-ray spectrum is different from the X-ray background spectrum due to the sensitivity of an instrument and other reasons. Before the Fermi gamma-ray space telescope era, neither spectrum nor origin of the EGRB was well understood. In particular, the spectrum at 0.03–50GeV reported by EGRET has a break in the several GeV. With the arrival of Fermi era, more accurate determination of the EGRB spectrum and more extragalactic source samples are provided to understand the nature of the EGRB. Note that the whole gamma-ray sky contains diffuse galactic emission, point sources, isotropic extragalactic diffuse emission and local and solar diffuse emissions. Figure 1 (right panel) shows that the EGRB spectrum is obtained by removing the resolved point source, like as the most recent list of resolved Fermi-LAT source (3FGL), the diffuse galactic emission determined by GALPROP, which simulates both cosmic-ray propagation in the galaxy and the gamma-ray flux resulting from interactions and possibly an isotropic flux of galactic, by

Similar to the extragalactic EGRET sky, blazars are the largest source class identified by Fermi extragalactic sky and their contribution to the EGRB has been widely discussed. Typical estimated contributions of unresolved blazars to the EGRB range from 10 to 100% [13–36]. Blazars are divided into two main subgroups: BL Lac objects and FSRQs [37]. Among the gamma-ray blazar sample, the number of FSRQs detected by Fermi-LAT is smaller than that of BL Lac objects (e.g., 2FGL, 3FGL). FSRQs generally show softer spectrum in the gamma-ray band (e.g., [38]), which is to be detected harder than BL Lac objects at a given significant limit. On the one hand, BL Lacs are reputed as the population of extragalactic sources that show a negative or no cosmological evolution [39–42], but FSRQs are regarded as those with a positive cosmological evolution, which

Figure 1. Left: The measured X-ray and gamma-ray background radiation spectra, which is from Ref. [5]. Right: The composition of the total gamma-ray flux. The figure is obtained from the report of Ackermann, M. at Fermi Symposium.

restricting data to regions with |b|>10° or even higher galactic latitudes.

90 New Insights on Gamma Rays

As professed in Ref. [31], there is a classical approach to obtain the luminosity function, which is on account of 1/VMAX method provided by Schmidt [60] to deal with redshift bins. However, this method has a fault, which is known to introduce bias in each binning. For a small sample and/or a large span of parameters, if the bins contained significant evolution, the method would result in a loss of important information. In order to constrain the model parameters for various models of the evolving GLF, a maximum likelihood method is adopted, which is first introduced by Marshall et al. [61]. The likelihood function L is given as follows (e.g., [17, 19, 24, 62]):

$$\mathcal{L} = \exp\left(\mathrm{-N}\_{\mathrm{exp}}\right) \prod\_{i=1}^{N\_{ab}} \Phi(L\_{\gamma,i}, z\_i, \Gamma\_i), \tag{1}$$

where Nexp is the expected number of source detections:

Nexp ¼ ð dΓ ð dzð dLγΦðLγ,i, zi, ΓiÞ, Nexp is the number of the sample of sources and ΦðLγ,i, zi, ΓiÞ is the distribution function of the space density of source on luminosity (Lγ), redshift (z) and photon index (Γ). The function form can be expressed as follows:

$$\Phi(L\_{\gamma,i}, z\_i, \Gamma\_i) = \frac{d^3N}{d\Gamma\_{\gamma}d\mathbf{z}d\Gamma} = \rho\_{\gamma}(L\_{\gamma}, z) \times \frac{dN}{d\Gamma} \times \frac{dV}{dz} \times \omega(L\_{\gamma}, z, \Gamma),\tag{2}$$

where ργðLγ, zÞ is the γ-ray luminosity function and dV=dz is the comoving volume element per unit redshift and unit solid angle:

dV=dz <sup>¼</sup> cd<sup>2</sup> <sup>L</sup>=ðH0ð1 þ zÞ 2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΩMð1 þ zÞ <sup>3</sup> <sup>þ</sup> ΩΛ q Þ: dN=dΓ is the intrinsic photon index distribution assumed as a Gaussian expð−ðΓ−μÞ 2 <sup>=</sup>2σ<sup>2</sup>Þ, where <sup>μ</sup> and <sup>σ</sup> are the mean and the dispersion, respectively. ωðLγ, z, ΓÞ is the detection efficiency and represents the probability of detecting an object with the γ-ray luminosity L<sup>γ</sup> at redshift z and photon index Γ [1, 24, 31]. The relationship between <sup>χ</sup><sup>2</sup> and likelihood (L) can be expressed by function <sup>χ</sup><sup>2</sup> <sup>¼</sup> <sup>−</sup>2 ln (L) [63]. In this case, the function <sup>χ</sup><sup>2</sup> <sup>¼</sup> <sup>−</sup>2 ln <sup>L</sup> that is minimized is defined as follows:

$$\chi^2 = -2\sum\_{i}^{N\_{\rm do}} \ln(\mathcal{O}(L\_{\gamma,i}, z\_i, \Gamma\_i)) + 2N\_{\rm exp}.\tag{3}$$

For a given GLF, the redshift distribution, luminosity distribution and photon index distribution can be divided into three intervals of size dLγdzdΓ and the three kinds of differential distributions can be expressed from GLF as follows [31]:

$$\begin{split} \frac{dN}{dz} &= \int\_{\Gamma\_{\rm min}}^{\Gamma\_{\rm max}} \int\_{\gamma}^{d} \frac{d^3N}{dL\_{\gamma} dz d\Gamma} dL\_{\gamma} dz, \\ \frac{dN}{dz} &= \int\_{\Gamma\_{\rm min}}^{\Gamma\_{\rm max}} \int\_{\mathrm{d}L\_{\gamma}}^{3} \frac{d^3N}{dL\_{\gamma} dz d\Gamma} dz d\Gamma, \\ \frac{dN}{dz} &= \int\_{\Gamma\_{\rm max}}^{\Gamma\_{\rm max}} \int\_{\gamma}^{d} \frac{d^3N}{dL\_{\gamma} dz d\Gamma} dL\_{\gamma} dz, \end{split} \tag{4}$$

The source count distribution can be derived as follows:

$$\begin{split} \mathbf{N} \left( \boldsymbol{>} \boldsymbol{S} \right) &= \int\_{\Gamma\_{\min}}^{\Gamma\_{\max}} d\Gamma \int\_{z\_{\min}}^{z\_{\max}} d\boldsymbol{z} \int\_{L\_{\gamma}(\mathbf{z}, \boldsymbol{S})}^{L\_{\gamma}, \max} d\boldsymbol{L}\_{\gamma} \, d\boldsymbol{z} d\boldsymbol{\Gamma} \\ &= \int\_{\Gamma\_{\min}}^{\Gamma\_{\max}} \frac{dN}{d\Gamma} d\Gamma \int\_{z\_{\min}}^{z\_{\max}} \frac{dV}{dz} \bigg|\_{L\_{\gamma}(\mathbf{z}, \boldsymbol{S})}^{L\_{\gamma}, \max} \rho\_{\gamma}(\mathcal{L}\_{\gamma}, \boldsymbol{z}) \boldsymbol{\omega}(\mathcal{L}\_{\gamma}, \boldsymbol{z}, \boldsymbol{\Gamma}) d\mathcal{L}\_{\gamma} \end{split} \tag{5}$$

where Lγðz, SÞ is the luminosity of a source at redshift z with a flux of S<sup>γ</sup> (>100MeV).

Through minimized Eq. (3), we can obtain the best-fitting parameters of the models. There are multiple parameters in our various models to find the best in observational data in a multidimensional model parameter space; the MCMC technique can be employed for its high efficiency to constrain the model parameters. In this method, the Metropolis-Hastings algorithm that generates samples from the posterior distribution using a Markov Chain is used when sampling the model parameters and the probability density distributions of the model parameters are asymptotically proportional to the number density of the sample points. For each parameter set <sup>P</sup>, one obtains the likelihood function <sup>L</sup>ðPÞ∝exp <sup>−</sup>χ<sup>2</sup>ðPÞ=<sup>2</sup> , where χ<sup>2</sup> is obtained by comparing model predictions with observations. A new set of parameter P 0 is adopted to replace the existing one <sup>P</sup> with a probability of min {1;LðP′ Þ=LðPÞ}. The MCMC method has been reviewed by Fan et al.[64] and described in detail by Neal [65], Gamerman [66], Lewis and Bridle [67], Mackay [68].

#### 2.2. Models description

Nexp ¼

ð dΓ ð dz ð

92 New Insights on Gamma Rays

dV=dz <sup>¼</sup> cd<sup>2</sup>

dLγΦðLγ,i, zi, ΓiÞ, Nexp is the number of the sample of sources and ΦðLγ,i, zi, ΓiÞ

dN dΓ · dV

dz · <sup>ω</sup>ðLγ, <sup>z</sup>, <sup>Γ</sup>Þ, (2)

(4)

(5)

Þ: dN=dΓ is the intrinsic photon index distribution

<sup>=</sup>2σ<sup>2</sup>Þ, where <sup>μ</sup> and <sup>σ</sup> are the mean and the dispersion,

lnðΦðLγ,i, zi, ΓiÞÞ þ 2Nexp: (3)

is the distribution function of the space density of source on luminosity (Lγ), redshift (z) and

where ργðLγ, zÞ is the γ-ray luminosity function and dV=dz is the comoving volume element

respectively. ωðLγ, z, ΓÞ is the detection efficiency and represents the probability of detecting an object with the γ-ray luminosity L<sup>γ</sup> at redshift z and photon index Γ [1, 24, 31]. The relationship between <sup>χ</sup><sup>2</sup> and likelihood (L) can be expressed by function <sup>χ</sup><sup>2</sup> <sup>¼</sup> <sup>−</sup>2 ln (L) [63]. In this case, the

For a given GLF, the redshift distribution, luminosity distribution and photon index distribution can be divided into three intervals of size dLγdzdΓ and the three kinds of differential

> d3 N dLγdzd<sup>Γ</sup> dLγdz,

d3 N dLγdzd<sup>Γ</sup> dzdΓ,

> d3 N dLγdzd<sup>Γ</sup> dLγdz,

> > dL<sup>γ</sup>

ργðLγ, zÞωðLγ, z, ΓÞdL<sup>γ</sup>

d3 N dLγdzdΓ

dLγdzd<sup>Γ</sup> <sup>¼</sup> ργðLγ, <sup>z</sup><sup>Þ</sup> ·

photon index (Γ). The function form can be expressed as follows:

N

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΩMð1 þ zÞ

2

X Nobs

i

ð Γmax

ð Lγ,max

Lγ,min

ðzmax

zmin

ð Lγ,max

Lγ,min

Γmin

ð Γmax

Γmin

ð Γmax

Γmin

ð<sup>Γ</sup>max Γmin dΓ ð<sup>z</sup>max zmin dz ð<sup>L</sup>γ, max Lγðz,SÞ

ð<sup>z</sup>max zmin

dV dz

where Lγðz, SÞ is the luminosity of a source at redshift z with a flux of S<sup>γ</sup> (>100MeV).

ð<sup>L</sup>γ, max Lγðz,SÞ

Through minimized Eq. (3), we can obtain the best-fitting parameters of the models. There are multiple parameters in our various models to find the best in observational data in a

<sup>3</sup> <sup>þ</sup> ΩΛ

<sup>Φ</sup>ðLγ,i, zi, <sup>Γ</sup>iÞ ¼ <sup>d</sup><sup>3</sup>

2

q

function <sup>χ</sup><sup>2</sup> <sup>¼</sup> <sup>−</sup>2 ln <sup>L</sup> that is minimized is defined as follows:

distributions can be expressed from GLF as follows [31]:

The source count distribution can be derived as follows:

ð<sup>Γ</sup>max Γmin

¼

N ð> SÞ ¼

dN <sup>d</sup><sup>Γ</sup> <sup>d</sup><sup>Γ</sup>

<sup>χ</sup><sup>2</sup> <sup>¼</sup> <sup>−</sup><sup>2</sup>

dN dz ¼

dN dz ¼

dN dz ¼

per unit redshift and unit solid angle:

<sup>L</sup>=ðH0ð1 þ zÞ

assumed as a Gaussian expð−ðΓ−μÞ

The GLF models for different source classes are uncertainty. Currently, there are two methods for constructing the blazars' GLF: the first method is to build the GLF by assuming a relationship between the GLF and the luminosity function in a lower energy band, for example, that the GLF relates to radio luminosity function (RLF) or to the X-ray luminosity function (XLF) (e.g., [14, 16, 17, 19, 23, 28, 48, 69–72]); the second method is to construct the GLF directly using observed gamma-ray data of blazars (e.g., [15, 17, 22]). Before the Fermi era, constructing the GLF model indirectly was used more frequently due to the small EGRET samples, which results in blazar's contribution between the range of 10 and 100%. In next sections, we briefly review those models for directly constructing the GLF.

#### 2.2.1. The pure density evolution

The pure density evolution (PDE) model is the simplest scenario of evolution and the GLF has a following form:

$$\rho(L\_{\mathcal{V}}, z) = \frac{A}{\ln(10) L\_{\mathcal{V}}} \left[ \left( \frac{L\_{\mathcal{V}}}{L} \right)^{\mathcal{V}\_1} + \left( \frac{L\_{\mathcal{V}}}{L} \right)^{\mathcal{V}\_2} \right]^{-1} \times e(z), \tag{6}$$

where eðzÞ¼ð1 þ zÞ <sup>κ</sup> is the standard power-law evolutionary factor. In this case, there are five model parameters and other two parameters, μ and σ, are also added.

#### 2.2.2. The pure luminosity evolution

In the pure luminosity evolution (PLE) model, the GLF can be expressed as follows:

$$\rho(L\_{\mathcal{V}}, z) = \frac{A(1+z)^{\kappa}e^{z/\zeta}}{\ln(10)L\_{\mathcal{V}}} \left[ \left( \frac{L\_{\mathcal{V}}}{L\_{\ast}(1+z)^{\kappa}e^{z/\zeta}} \right)^{\gamma\_1} + \left( \frac{L\_{\mathcal{V}}}{L\_{\ast}(1+z)^{\kappa}e^{z/\zeta}} \right)^{\gamma\_2} \right]^{-1},\tag{7}$$

where A is a normalization factor, Li is the evolving break luminosity, γ<sup>1</sup> is the faint-end slope index, γ<sup>2</sup> is the bright-end slope index, κ and ξ represent the redshift evolution. Including the parameters μ and σ, there are 8 parameters in calculations.

#### 2.2.3. The luminosity-dependent density evolution

In the luminosity-dependent density evolution (LDDE) model, the GLF evolution is decided by a redshift cutoff that depends on luminosity and the GLF can be given by

$$\rho(\mathcal{L}\_{\mathcal{V}}, z) = \frac{A}{\ln(10) L\_{\mathcal{V}}} \left[ \left( \frac{L\_{\mathcal{V}}}{L\_{\ast}} \right)^{\mathcal{V}\_{1}} + \left( \frac{L\_{\mathcal{V}}}{L\_{\ast}} \right)^{\mathcal{V}\_{2}} \right]^{-1}$$

$$\left[ \left( \frac{1 + z}{1 + z\_{c}^{\*} (L\_{\mathcal{V}} / 10^{48})^{a}} \right)^{p\_{1}} + \left( \frac{1 + z}{1 + z\_{c}^{\*} (L\_{\mathcal{V}} / 10^{48})^{a}} \right)^{p\_{2}} \right], \tag{8}$$

where A is a normalization factor, L is evolving break luminosity, γ<sup>1</sup> and p<sup>1</sup> are the faint-end slope index, γ<sup>2</sup> and p<sup>2</sup> are the bright-end slope index, zc is redshift peak with a luminosity (here 1048 ergs s<sup>−</sup>1) and α is power-law index of the redshift-peak evolution. From this, there are 10 parameters for calculation.

The detailed description about PLE and LDDE models can be found in sections4.1 and 4.2 from Ref. [32]. These models also can be applied to X-ray band, to determine the information of evolution of sources in X-ray band (e.g., [62]). With the increase in the number of the detected sources, the evolutionary form of those sources becomes more complicated and the updated forms of those models can be found in Ref. [33], which allows the Gaussian mean μ of the photon index and the evolutionary factor eðz, LγÞ to change with luminosity.

#### 2.3. The cosmological evolution

In Fermi sample, the large redshift range between z ¼ 0 and z ¼ 3:1 of gamma-ray blazars was found. The obtained GLFs have shown that blazars have a cosmological evolution in their gamma-ray band. We have simply discussed the redshift evolution of blazars in the "Introduction". Ajello et al. [32] recently have presented the new results on the cosmological evolution of the BL Lac population by using the largest and most complete sample of gamma-ray BL Lacs available in the literature and they found that for most BL Lac classes, the evolution is positive, with a space density peaking at modest redshift (z≈1.2) (see Figure 2). In Figure 2, we also see that for their higher luminosity, FSRQs dominate at all redshifts z>0.3 and the extreme growth in BL Lac numbers at low z allows them to produce ~90% of the local luminosity density. In particular, low-luminosity, high-synchrotron-peaked (HSP) BL Lac objects showed different evolutionary behaviors with respect to other blazar classes (see Figure 2). They have strong

Figure 2. Left: The evolution of the luminosity density of FSRQs compared to that of BL Lac objects. Right: Number density of FSRQs, BL Lac objects and HSPs. The figures are obtained from the report of [32] and see Ref. [32] for additional details.

negative evolution with number density increasing for z<0.5, which confirms previous standpoints of negative evolution based on the samples of X-ray-selected BL Lac objects and this sample contained a large fraction of HSPs [39, 41].

#### 3. The extragalactic gamma-ray background

<sup>ρ</sup>ðLγ, <sup>z</sup>Þ ¼ <sup>A</sup>

parameters for calculation.

94 New Insights on Gamma Rays

2.3. The cosmological evolution

details.

lnð10ÞL<sup>γ</sup>

1 þ z�

Lγ L� � �<sup>γ</sup><sup>1</sup>

1 þ z

!<sup>p</sup><sup>1</sup>

<sup>c</sup> <sup>ð</sup>Lγ=1048<sup>Þ</sup>

photon index and the evolutionary factor eðz, LγÞ to change with luminosity.

þ Lγ L�

α

� �<sup>γ</sup><sup>2</sup> � �<sup>−</sup><sup>1</sup>

þ

where A is a normalization factor, L is evolving break luminosity, γ<sup>1</sup> and p<sup>1</sup> are the faint-end slope index, γ<sup>2</sup> and p<sup>2</sup> are the bright-end slope index, zc is redshift peak with a luminosity (here 1048 ergs s<sup>−</sup>1) and α is power-law index of the redshift-peak evolution. From this, there are 10

The detailed description about PLE and LDDE models can be found in sections4.1 and 4.2 from Ref. [32]. These models also can be applied to X-ray band, to determine the information of evolution of sources in X-ray band (e.g., [62]). With the increase in the number of the detected sources, the evolutionary form of those sources becomes more complicated and the updated forms of those models can be found in Ref. [33], which allows the Gaussian mean μ of the

In Fermi sample, the large redshift range between z ¼ 0 and z ¼ 3:1 of gamma-ray blazars was found. The obtained GLFs have shown that blazars have a cosmological evolution in their gamma-ray band. We have simply discussed the redshift evolution of blazars in the "Introduction". Ajello et al. [32] recently have presented the new results on the cosmological evolution of the BL Lac population by using the largest and most complete sample of gamma-ray BL Lacs available in the literature and they found that for most BL Lac classes, the evolution is positive, with a space density peaking at modest redshift (z≈1.2) (see Figure 2). In Figure 2, we also see that for their higher luminosity, FSRQs dominate at all redshifts z>0.3 and the extreme growth in BL Lac numbers at low z allows them to produce ~90% of the local luminosity density. In particular, low-luminosity, high-synchrotron-peaked (HSP) BL Lac objects showed different evolutionary behaviors with respect to other blazar classes (see Figure 2). They have strong

Figure 2. Left: The evolution of the luminosity density of FSRQs compared to that of BL Lac objects. Right: Number density of FSRQs, BL Lac objects and HSPs. The figures are obtained from the report of [32] and see Ref. [32] for additional

!<sup>p</sup><sup>2</sup> " #

1 þ z

<sup>c</sup> <sup>ð</sup>Lγ=1048<sup>Þ</sup>

α

, (8)

1 þ z�

The origin of the EGRB has been widely discussed for various gamma-ray-emitting sources in literature. Fermi has observed gamma-ray emission from blazars, star-forming galaxies, radio galaxies, GRBs and high-latitude pulsars. Ajello et al. [33] and Di Mauro and Donato [36] suggested that blazars, star-forming galaxies and radio galaxies are the main contributors to the EGRB. For those emitting sources, we focus on how to estimate the contribution of unresolved objects to the EGRB below, based on the best-fitting GLF (space density of sources).

The differential intensity of the EGRB radiation can be expressed as follows:

$$\begin{split} \frac{dN}{d\mathbb{E}d\varOmega} &= \int\_{\varGamma\_{\min}} d\varGamma \frac{dN}{d\varGamma} \int\_{z\_{\min}}^{z\_{\max}} \int\_{L\_{\gamma,\min}}^{L\_{\gamma,\max}} dL\_{\gamma} \\ \upU\_{\varGamma}(L\_{\varGamma}, z) F\_{\varGamma}^{intraisic}(\mathbb{E}, L\_{\varGamma}, z, \varGamma) e^{-\tau(\varGamma, z)} \Big(1.0 - \omega(L\_{\varGamma}, z, \varGamma)\Big) \end{split} \tag{9}$$

where <sup>Φ</sup>ðLγ, <sup>z</sup><sup>Þ</sup> is the GLF and <sup>e</sup><sup>−</sup>τðE, <sup>z</sup><sup>Þ</sup> is the optical depth of the extragalactic background light (EBL) for the sources at redshift z emitting gamma-ray photon energy E. Recently, there are many studies on EBL (e.g., [21, 73–75]). Generally, we adopted the model given by [73] for the EBL to calculate the optical depth. In Eq. (9), Fintrinsic <sup>γ</sup> ðE, Lγ, z, ΓÞ represents the intrinsic photon flux at energy E with γ-ray luminosity L<sup>γ</sup> and a power-law spectrum at redshift z and it is expressed as follows:

$$F\_{\gamma}^{\text{intrinsic}}(\mathbf{E}, \ L\_{\gamma}, z, \Gamma) = \frac{L\_{\gamma} \left(1 + z\right)^{2 - \Gamma}}{4\pi d\_{L}^{2} E\_{1}^{2}} \left\{ \begin{array}{ll} \left(2 - \Gamma\right) \left[\left(\frac{E\_{2}}{E\_{1}}\right)^{2 - \Gamma} - 1\right]^{-1} \left(\frac{E}{100 \text{ MeV}}\right)^{-\Gamma} & \Gamma \neq 2, \\\left(\frac{1}{\ln(E\_{2}/E\_{1})} \left(1 - \frac{E\_{1}}{E\_{2}}\right)^{-1} \left(\frac{E}{100 \text{ MeV}}\right)^{-2} & \Gamma = 2, \end{array} \right. \\(10) \right\}$$

where E<sup>1</sup> ¼ 100 MeV and E<sup>2</sup> ¼ 100 GeV. Therefore, the integrated intensity between photon energy E<sup>1</sup> and E<sup>2</sup> ðE<sup>2</sup> > E1Þ can be written as follows:

$$\frac{dN}{d\Omega} = \int\_{E\_1}^{E\_2} \frac{dN}{dEd\Omega} dE \tag{11}$$

The electrons and positrons are produced due to the interaction between very high energy (VHE) photons from TeV sources and ultraviolet-infrared photons of EBL. The pairs could scatter the cosmic microwave background (CMB) radiation to high-energy background radiation through the inverse Compton scattering process (e.g., [76–83]). This cascading emission is regarded as a contributor to the EGRB if the flux of the cascade flux is lower than the detector's sensitivity. Now, we consider only the first generation of the electron-positron pairs produced by the gamma-ray absorption to obtain the cascade emission because the emission from the second generation or more than second generation of created pairs can be negligible at the GeV band [21]. The formulation of the cascade flux is given as follows [84]:

$$F\_{\gamma}^{\text{causal}}(E, L\_{\gamma}, z, \Gamma) = \frac{81}{16} \frac{\pi}{\lambda\_c^3} \frac{\varepsilon\_c^2 m\_c c^2}{(1+z)^4 L\_{\text{CMB}}}$$

$$\begin{split} \int & \qquad \frac{d\gamma}{\gamma^8 \exp[3\varepsilon\_c/4\gamma^2 \varepsilon\_{\text{CMB}}(1+z)-1]}\\ & \qquad \sqrt{\frac{3\varepsilon\_c/4\varepsilon\_{\text{CM}}(1+z)}{\varepsilon\_{\text{mix}}}}\\ & \qquad \times \int d\varepsilon \; F\_{\text{VHE}}^{\text{intrinsic}}(\frac{5.11 \times 10^5}{10^6} \varepsilon, z, L\_{\gamma}, \Gamma) [1 - e^{-\varepsilon(\varepsilon, z)}] \end{split} \tag{12}$$

where <sup>λ</sup><sup>c</sup> <sup>¼</sup> <sup>2</sup>:<sup>426</sup> · <sup>10</sup><sup>−</sup><sup>10</sup> cm is the Compton length, the dimensionless energy <sup>ε</sup><sup>c</sup> <sup>¼</sup> <sup>E</sup> · <sup>10</sup><sup>6</sup> = <sup>ð</sup>5:<sup>11</sup> · 105 <sup>Þ</sup>, UCMB <sup>¼</sup> <sup>4</sup>:<sup>0</sup> · <sup>10</sup><sup>−</sup><sup>13</sup> erg cm<sup>−</sup><sup>3</sup> is the CMB energy density at <sup>z</sup> <sup>¼</sup> <sup>0</sup>:0; <sup>ε</sup>CMB <sup>¼</sup> <sup>1</sup>:<sup>24</sup> · 109 mec<sup>2</sup> is the average CMB photon energy at <sup>z</sup> <sup>¼</sup> <sup>0</sup>:0 and <sup>ε</sup>CMB <sup>¼</sup> <sup>2</sup>:<sup>0</sup> · 108 corresponding to EVHE <sup>¼</sup> <sup>100</sup>TeV. Fintrinsic VHE ðEVHE, Lγ, z, ΓÞ represents the possible intrinsic TeV spectrum, which is extrapolated to the TeV energy ranges from the observed GeV spectrum Eq. (10) by assuming a power-law spectrum. In Eq. (9), using Eq. (12) in place of Eq. (10) allows us to compute the contribution to the EGRB from the cascade emission of the source.

It is noted that there are two possible contributions for the cascade emission to the EGRB because the pairs are deflected by the extragalactic magnetic field (EGMF), which is shown in Figure 3. In case I, the cascade emission can contribute to the EGRB if the flux of the cascade emission is lower than that of the LAT sensitivity. In case II, although the flux of the cascade emission is larger than that of the LAT sensitivity, the angle between the redirected secondary gamma-ray photons and the line of sight is larger than that of the LAT point-spread function (PSF) (i.e., θ > θPSF). Thus, the cascade emission will not be attributed to a point source by the LAT and it then contributes to the EGRB, where θPSF ¼ ð1:7π=180Þð0:001EÞ <sup>−</sup>0:<sup>74</sup>½<sup>1</sup> þ ð0:001E<sup>=</sup> 15Þ 2 � <sup>0</sup>:<sup>37</sup> [85]. For more detailed information, see Refs. [81, 84].

#### 3.1. Blazars

Blazars emit gamma rays via the inverse Compton scattering processes and/or hadronic processes and dominate extragalactic gamma-ray sources. Therefore, it is naturally expected that blazars contribute the main EGRB. However, its fraction was very uncertain in the EGRET era due to its small samples. At the same time, its fraction also severely depends on GLF. Blazars are divided into two main subgroups: BL Lac objects and FSRQs [37]. Figure 4 shows FSRQs' EGRB spectra with LDDE model and BL Lacs' EGRB spectra with PDE model. Compared to FSRQs, BL Lacs have lower gamma-ray luminosities, lower redshifts and harder spectral indices in statistics (e.g., [86]). Thus, BL Lacs can provide a significant part in the contribution of blazar to the EGRB above 10GeV. From Figure 4, we find out that the cascade emission from BL Lacs has a rather large fraction of the total EGRB energy flux and contrary to that of FSRQs, which may be caused by harder spectrum for BL Laces. Therefore, the contribution from BL Lacs cascade emission to the EGRB cannot be negligible. Based on the effect of the EGMF on the cascade contribution from blazars, Yan [84] have studied the effect of cascade radiation on the contribution to the EGRB using a simple semi-analytical model. They suggested that if the strength of the EGMF is large enough (BEGMF > 10<sup>−</sup><sup>12</sup> G), the cascade contribution can significantly alter the spectrum of the EGRB at high energies. If the small strength of the EGMF is large enough (BEGMF < 10<sup>−</sup><sup>14</sup> G), then the cascade contribution is small, but it cannot be ignored. Recently, Ajello et al. [33] used an updated GLFs to analyze the redshift, luminosity and photon index distributions and obtained the best-fitting evolutionary parameters of the GLFs. According to the GLFs and spectral energy distribution (SED) model consistent with the Fermi blazar observations, their result shown that blazars account for 50þ<sup>12</sup> −10 to the EGRB (see Figure 5).

radiation through the inverse Compton scattering process (e.g., [76–83]). This cascading emission is regarded as a contributor to the EGRB if the flux of the cascade flux is lower than the detector's sensitivity. Now, we consider only the first generation of the electron-positron pairs produced by the gamma-ray absorption to obtain the cascade emission because the emission from the second generation or more than second generation of created pairs can be negligible

> 16 λ<sup>3</sup> c

5:11 · 105

where <sup>λ</sup><sup>c</sup> <sup>¼</sup> <sup>2</sup>:<sup>426</sup> · <sup>10</sup><sup>−</sup><sup>10</sup> cm is the Compton length, the dimensionless energy <sup>ε</sup><sup>c</sup> <sup>¼</sup> <sup>E</sup> · <sup>10</sup><sup>6</sup>

mec<sup>2</sup> is the average CMB photon energy at <sup>z</sup> <sup>¼</sup> <sup>0</sup>:0 and <sup>ε</sup>CMB <sup>¼</sup> <sup>2</sup>:<sup>0</sup> · 108 corresponding to

extrapolated to the TeV energy ranges from the observed GeV spectrum Eq. (10) by assuming a power-law spectrum. In Eq. (9), using Eq. (12) in place of Eq. (10) allows us to compute the

It is noted that there are two possible contributions for the cascade emission to the EGRB because the pairs are deflected by the extragalactic magnetic field (EGMF), which is shown in Figure 3. In case I, the cascade emission can contribute to the EGRB if the flux of the cascade emission is lower than that of the LAT sensitivity. In case II, although the flux of the cascade emission is larger than that of the LAT sensitivity, the angle between the redirected secondary gamma-ray photons and the line of sight is larger than that of the LAT point-spread function (PSF) (i.e., θ > θPSF). Thus, the cascade emission will not be attributed to a point source by the

Blazars emit gamma rays via the inverse Compton scattering processes and/or hadronic processes and dominate extragalactic gamma-ray sources. Therefore, it is naturally expected that blazars contribute the main EGRB. However, its fraction was very uncertain in the EGRET era due to its small samples. At the same time, its fraction also severely depends on GLF. Blazars are divided into two main subgroups: BL Lac objects and FSRQs [37]. Figure 4 shows FSRQs' EGRB spectra with LDDE model and BL Lacs' EGRB spectra with PDE model. Compared to FSRQs, BL Lacs have lower gamma-ray luminosities, lower redshifts and harder spectral indices in statistics (e.g., [86]). Thus, BL Lacs can provide a significant part in the

dγ γ8exp½3εc=4γ<sup>2</sup>εCMBð1 þ zÞ−1�

<sup>106</sup> <sup>ε</sup>, <sup>z</sup>, <sup>L</sup>γ, <sup>Γ</sup> � �

<sup>Þ</sup>, UCMB <sup>¼</sup> <sup>4</sup>:<sup>0</sup> · <sup>10</sup><sup>−</sup><sup>13</sup> erg cm<sup>−</sup><sup>3</sup> is the CMB energy density at <sup>z</sup> <sup>¼</sup> <sup>0</sup>:0; <sup>ε</sup>CMB <sup>¼</sup> <sup>1</sup>:<sup>24</sup> ·

VHE ðEVHE, Lγ, z, ΓÞ represents the possible intrinsic TeV spectrum, which is

ε2 cmec<sup>2</sup>

> ½1−e −τðε, zÞ

� (12)

<sup>−</sup>0:<sup>74</sup>½<sup>1</sup> þ ð0:001E<sup>=</sup>

=

ð1 þ zÞ 4 UCMB

at the GeV band [21]. The formulation of the cascade flux is given as follows [84]:

<sup>γ</sup> <sup>ð</sup>E, <sup>L</sup>γ, <sup>z</sup>, <sup>Γ</sup>Þ ¼ <sup>81</sup> <sup>π</sup>

Fcascade

· εð max

<sup>ð</sup>5:<sup>11</sup> · 105

EVHE <sup>¼</sup> <sup>100</sup>TeV. Fintrinsic

96 New Insights on Gamma Rays

109

15Þ 2 �

3.1. Blazars

2γ

ð ∞

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>3</sup>εc=4εCMBð1þz<sup>Þ</sup> <sup>p</sup>

contribution to the EGRB from the cascade emission of the source.

LAT and it then contributes to the EGRB, where θPSF ¼ ð1:7π=180Þð0:001EÞ

<sup>0</sup>:<sup>37</sup> [85]. For more detailed information, see Refs. [81, 84].

dε Fintrinsic VHE

Figure 3. The cascade radiation processes in no or non-zero extragalactic magnetic field (EGMF). Note that the pairs produced by the interaction between very high energy (VHE) photons and ultraviolet-infrared photons of EBL are detected by the EGMF. The figures are obtained from the report of Marco Ajello at Fermi Symposium.

Figure 4. Comparison of predicted EGRB spectra from FSRQs and BL Lacs with the observed data of blazars. Note that the EGRB spectra from FSRQs and BL Lac are estimated based on LDDE and PDE models, respectively. The two figures are obtained from the report of Refs. [29, 30].

Figure 5. The EGRB spectrum of blazars [33], star-forming galaxies (gray band [4]) and radio galaxies (black striped band [48]) as well as summation of these three populations (yellow band), compared to the intensity of the observed ERGB [5]. The figure is obtained from the report of Ref. [33].

#### 3.2. Radio galaxies

Radio galaxies are one of the largest subclasses of radio-loud AGNs. It is more in number than blazars in the entire sky. Even though radio galaxies are fainter than blazars, Fermi-LAT has just detected gamma rays from ~15 extragalactic sources, including 12 FR Is and 3 FR IIs [87]. In order to estimate the contribution to the EGRB from radio galaxies, their GLF is required. We must obtain indirectly the GLF due to the limited Fermi radio galaxy samples. Relying on a correlation between the luminosities in the radio and gamma-ray frequencies, Inoue [48] converted the RLF [88] into the GLF and estimated about 25% of EGRB can be solved by radio galaxies (see Figure 5). This uncertainty significantly depends on the limited sample and the errors between the gamma ray and radio luminosity correlation.

#### 3.3. Star-forming galaxies

Figure 4. Comparison of predicted EGRB spectra from FSRQs and BL Lacs with the observed data of blazars. Note that the EGRB spectra from FSRQs and BL Lac are estimated based on LDDE and PDE models, respectively. The two figures

Figure 5. The EGRB spectrum of blazars [33], star-forming galaxies (gray band [4]) and radio galaxies (black striped band [48]) as well as summation of these three populations (yellow band), compared to the intensity of the observed ERGB [5].

are obtained from the report of Refs. [29, 30].

98 New Insights on Gamma Rays

The figure is obtained from the report of Ref. [33].

The Fermi-LAT has detected gamma-ray from ~9 star-forming (SF) galaxies [2]. Those gamma rays are produced by interactions between cosmic rays and gas or interstellar radiation fields, including the decay of neutral pion and electron interactions (bremsstrahlung and inverse Compton scattering). Similar to radio galaxies, it is not straightforward to construct the GLF because of the limited star-forming galaxy sample. Generally, the correlations between the IR wavelength and gamma-ray region are used to predict the gamma-ray diffuse emission for the unresolved SF galaxy population. Different from other types of source, the SF gamma-ray average spectrum is difficult to firmly establish due to the paucity of statistics. Milky Way-like SF galaxies (MW model) and an assumed power-law spectrum (PL model) are proposed by Ackermann et al. [89] to express an average spectrum of SF Galaxies. In particular, the two predictions are different above 5GeV, where the MW model softens significantly. Therefore, using the correlation between infrared and gamma-ray luminosities, based on the wellestablished infrared luminosity functions and the SF gamma-ray average spectrum, the GLF of star-forming galaxies is well built and the contribution of star-forming galaxies to the EGRB can be estimated as 10–30% of the EGRB at >0.1GeV [89], which can be seen in Figure 5.

It should be noted that about 95% of the EGRB can be naturally explained by blazars, starforming galaxies and radio galaxies in the 0.1–820GeV range. Only modest space is left for other diffuse processes such as dark matter interactions, which suggests that other gammaray-emitting sources' contribution can be neglected. Ajello et al. [33] also concluded that the result of their simulation gave an upper limit on DM self-annihilation cross sections, which is similar to that from the independent types of analysis (e.g., [59]).

#### 4. Conclusion and discussion

In this chapter, we reviewed the origin of EGRB and estimated the contribution of unsolved gamma-ray-emitting sources from Fermi-LAT to the EGRB based on the construction of the corresponding GLFs. Since Fermi-LAT has higher sensitivity and provides numerous gammaray-emitting sources for studies, we found two important results: (i) the redshift evolutionary information of gamma-ray sources, particularly for blazars; HBLs show strong negative cosmological evolution, while FSRQs and luminous BL Lacs show positive evolution like as Seyferts and the cosmic star formation history. (ii) Fermi sources' contribution to the EGRB; blazars clearly contribute to most of the EGRB (≈40-62%), as well as radio galaxies and starforming galaxies can occupy for the rest room of the EGRB [33, 36]. These results suggest that the contributions of other emitting sources have only little space to the EGRB. However, the uncertainties associated with these predictions from radio galaxies and star-forming galaxies are still quite large because of the small samples. This situation is very similar to blazar studies in the early EGRET era. Therefore, further data will be required to construct the GLFs and precisely evaluate the contributions from those two populations.

Now, there are still some unresolved problems. We have not seen the signature of dark matter particles in the EGRB spectrum, although they are considered as the possible origin of EGRB. As we known, Fermi-LAT has accurately measured the EGRB spectrum and the anisotropy of the EGRB [4] and the emission from dark matter is anisotropic and its spatial pattern is unique and predictable [90]. Therefore, we can obtain an upper limit on the annihilation cross section by comparing the expected EGRB angular power spectrum from dark matter annihilation with the measured spectrum. The work of Ajello et al. [33] shown that an analysis of the EGRB and its components can constrain diffuse emission mechanisms such as DM annihilation. Di Mauro and Donato [36] probed a possible emission coming from the annihilation of WIMP DM in the halo of our galaxy and found that the DM component can very well fit the EGRB data together with the realistic emission from a number of unresolved extragalactic sources.

The value of the EGMF has still not been determined. Since the pairs scatter CMB photons to GeV energies by Compton mechanism for cascade process around a TeV sources, Fermi-LAT could measure those GeV photons, which would give a straight measurement of the EGMF. As continuous accumulation of the data observed and the further development of detection equipment, the imprint of the EGMF may be found in the gamma-ray spectrum and/or flux [79, 80, 91]. The EGMF imprint might also be found in the angular anisotropy of the EGRB [92]. If the effect of cascade depending on the EGMF cannot be neglected [84], the electron-positron pairs produced in cascade process could be deflected by a high value of the EGMF, which makes GeV photons more isotropic. Therefore, the EGRB spectrum with the anisotropy could probe the strength of EGMF [87].

#### Author details

Houdun Zeng1,3 and Li Zhang2,3\*

\*Address all correspondence to: lizhang@ynu.edu.cn

1 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China

2 Department of Astronomy, Yunnan University, Kunming, China

3 Key Laboratory of Astroparticle Physics of Yunnan Province, Kunming, China

#### References

cosmological evolution, while FSRQs and luminous BL Lacs show positive evolution like as Seyferts and the cosmic star formation history. (ii) Fermi sources' contribution to the EGRB; blazars clearly contribute to most of the EGRB (≈40-62%), as well as radio galaxies and starforming galaxies can occupy for the rest room of the EGRB [33, 36]. These results suggest that the contributions of other emitting sources have only little space to the EGRB. However, the uncertainties associated with these predictions from radio galaxies and star-forming galaxies are still quite large because of the small samples. This situation is very similar to blazar studies in the early EGRET era. Therefore, further data will be required to construct the GLFs and

Now, there are still some unresolved problems. We have not seen the signature of dark matter particles in the EGRB spectrum, although they are considered as the possible origin of EGRB. As we known, Fermi-LAT has accurately measured the EGRB spectrum and the anisotropy of the EGRB [4] and the emission from dark matter is anisotropic and its spatial pattern is unique and predictable [90]. Therefore, we can obtain an upper limit on the annihilation cross section by comparing the expected EGRB angular power spectrum from dark matter annihilation with the measured spectrum. The work of Ajello et al. [33] shown that an analysis of the EGRB and its components can constrain diffuse emission mechanisms such as DM annihilation. Di Mauro and Donato [36] probed a possible emission coming from the annihilation of WIMP DM in the halo of our galaxy and found that the DM component can very well fit the EGRB data together

The value of the EGMF has still not been determined. Since the pairs scatter CMB photons to GeV energies by Compton mechanism for cascade process around a TeV sources, Fermi-LAT could measure those GeV photons, which would give a straight measurement of the EGMF. As continuous accumulation of the data observed and the further development of detection equipment, the imprint of the EGMF may be found in the gamma-ray spectrum and/or flux [79, 80, 91]. The EGMF imprint might also be found in the angular anisotropy of the EGRB [92]. If the effect of cascade depending on the EGMF cannot be neglected [84], the electron-positron pairs produced in cascade process could be deflected by a high value of the EGMF, which makes GeV photons more isotropic. Therefore, the EGRB spectrum with the anisotropy could probe the strength of EGMF [87].

1 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory,

3 Key Laboratory of Astroparticle Physics of Yunnan Province, Kunming, China

precisely evaluate the contributions from those two populations.

with the realistic emission from a number of unresolved extragalactic sources.

Author details

100 New Insights on Gamma Rays

Houdun Zeng1,3 and Li Zhang2,3\*

\*Address all correspondence to: lizhang@ynu.edu.cn

2 Department of Astronomy, Yunnan University, Kunming, China

Chinese Academy of Sciences, Nanjing, China


[26] Stecker, F. W.; Venters, T. M. Components of the extragalactic gamma-ray background. The Astrophysical Journal. 2011;736(1):40, 13 pp. doi:10.1088/0004-637X/736/1/40

[13] Stecker, F. W.; Salamon, M. H.; Malkan, M. A. The high-energy diffuse cosmic gamma-ray background radiation from blazars. The Astrophysical Journal. 1993;410(2):L71–L74.

[14] Padovani, P.; Ghisellini, G.; Fabian, A. C.; Celotti, A. Radio-loud AGN and the extragalactic gamma-ray background. Monthly Notices of the Royal Astronomical Society.

[15] Chiang, J.; Fichtel, C. E.; von Montigny, C.; Nolan, P. L.; Petrosian, V. The evolution of gamma-ray--loud active galactic nuclei. The Astrophysical Journal. 1995;452:165. doi:10.

[16] Stecker, F. W.; Salamon, M. H. The gamma-ray background from blazars: a new look. The

[17] Chiang, J.; Mukherjee, R. The luminosity function of the EGRET gamma-ray blazars. The

[18] Mücke, A.; Pohl, M. The contribution of unresolved radio-loud AGN to the extragalactic diffuse gamma-ray background. Monthly Notices of the Royal Astronomical Society.

[19] Narumoto, T.; Totani, T. Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution. The

[20] Dermer, C. D. Statistics of cosmological black hole jet sources: blazar predictions for the gamma-ray large area space telescope. The Astrophysical Journal. 2007;659(2):958–975.

[21] Kneiske, T. M.; Mannheim, K. BL Lacertae contribution to the extragalactic gamma-ray background. Astronomy and Astrophysics. 2008;479(1):41–47. doi:10.1051/0004-6361:200

[22] Bhattacharya, D.; Sreekumar, P.; Mukherjee, R. Gamma-ray luminosity function of gamma-ray bright AGNs. Research in Astronomy and Astrophysics. 2008;9(1):85–94.

[23] Inoue, Y.; Totani, T. The blazar sequence and the cosmic gamma-ray background radiation in the Fermi era. The Astrophysical Journal. 2009;702(1):523–536. doi:10.1088/0004-

[24] Abdo, A. A.; Ackermann, M.; Ajello, M.; Antolini, E.; Baldini, L.; Ballet, J.; et al. The Fermi-LAT high-latitude survey: source count distributions and the origin of the extragalactic diffuse background. The Astrophysical Journal. 2010;720(1):435–453. doi:10.1088/

[25] Ghirlanda, G.; Ghisellini, G.; Tavecchio, F.; Foschini, L.; Bonnoli, G. The radio-γ-ray connection in Fermi blazars. Monthly Notices of the Royal Astronomical Society.

2011;413(2):852–862. doi:10.1111/j.1365-2966.2010.18173.x

1993;260(3): L21–L24. doi:10.1093/mnras/260.1.L21

Astrophysical Journal. 1996;464:600. doi:10.1086/177348

2000;312(1):177–193. doi:10.1046/j.1365-8711.2000.03099.x

Astrophysical Journal. 2006;634(1):81–91. doi:10.1086/502708

Astrophysical Journal. 1998;496(2):752–760. doi:10.1086/305403

doi:10.1086/186882

102 New Insights on Gamma Rays

1086/176287

doi:10.1086/512533

637X/702/1/523

0004-637X/720/1/435

doi:10.1088/1674-4527/9/1/007

65605


[53] Inoue, Y.; Totani, T.; Ueda, Y. The cosmic MeV gamma-ray background and hard x-ray spectra of active galactic nuclei: implications for the origin of hot AGN coronae. The Astrophysical Journal Letters. 2008;672(1): L5. doi:10.1086/525848

[39] Rector, Travis A.; Stocke, John T.; Perlman, Eric S.; Morris, Simon L.; Gioia, Isabella M. The properties of the x-ray-selected EMSS sample of BL lacertae objects. The Astronom-

[40] Caccianiga, A.; Maccacaro, T.; Wolter, A.; Della Ceca, R.; Gioia, I. M. On the cosmological evolution of BL lacertae objects. The Astrophysical Journal. 2002;566(1):181–186. doi:10.10

[41] Beckmann, V.; Engels, D.; Bade, N.; Wucknitz, O. The HRX-BL Lac sample—evolution of BL lac objects. Astronomy and Astrophysics. 2003;401:927–938. doi:10.1051/0004-6361:

[42] Padovani, P.; Giommi, P.; Landt, H.; Perlman, Eric S. The deep X-ray radio blazar survey. III. Radio number counts, evolutionary properties and luminosity function of blazars.

[43] Dunlop, J. S.; Peacock, J. A. The redshift cut-off in the luminosity function of radio galaxies and quasars. Monthly Notices of the Royal Astronomical Society. 1990;247(1):19.

[44] Ueda, Y.; Akiyama, M.; Ohta, K.; Miyaji, T. Cosmological evolution of the hard x-ray active galactic nucleus luminosity function and the origin of the hard x-ray background.

[45] Hasinger, G.; Miyaji, T.; Schmidt, M. Luminosity-dependent evolution of soft X-ray selected AGN. New Chandra and XMM-Newton surveys. Astronomy and Astrophysics.

[46] Strong, A. W.; Wolfendale, A. W.; Worrall, D. M.. Origin of the diffuse gamma ray background. Monthly Notices of the Royal Astronomical Society. 1976;175:23–27. doi:10.

[47] Pavlidou, V.; Fields, B. D. The guaranteed gamma-ray background. The Astrophysical

[48] Inoue, Y. Contribution of gamma-ray-loud radio galaxies' core emissions to the cosmic MeV and GeV gamma-ray background radiation. The Astrophysical Journal. 2011;733

[49] Casanova, S.; Dingus, B. L.; Zhang, B. Contribution of GRB emission to the GeV extragalactic diffuse gamma-ray flux. The Astrophysical Journal. 2007;656(1):306–312. doi:10.

[50] Faucher-Giguère, Claude-André; Loeb, Abraham. The pulsar contribution to the gammaray background. Journal of Cosmology and Astroparticle Physics. 2010;01(01):005.

[51] Loeb, A.; Waxman, E. Cosmic γ-ray background from structure formation in the interga-

[52] Totani, T.; Kitayama, T. Forming clusters of galaxies as the origin of unidentified GEV gamma-ray sources. The Astrophysical Journal. 2000;545(2):572–577. doi:10.1086/317872

The Astrophysical Journal. 2007;662(1):182–198. doi:10.1086/516815

The Astrophysical Journal. 2003;598(2):886–908. doi:10.1086/378940

2005;441(2):417–434. doi:10.1051/0004-6361:20042134

Journal. 2002;575(1):L5–L8. doi:10.1086/342670

(1):66, 9 pp. doi:10.1088/0004-637X/733/1/66

lactic medium. Nature. 2000;405(6783):156–158.

doi:10.1088/1475-7516/2010/01/005

ical Journal. 2000;120(4):1626–1647. doi:10.1086/301587

86/338073

104 New Insights on Gamma Rays

20030184

1093/mnras/175.1.23P

1086/510613


blazar 1ES0229+200. Monthly Notices of the Royal Astronomical Society: Letters. 2010;406(1):L70–L74. doi:10.1111/j.1745-3933.2010.00884.x

[81] Dermer, C. D.; Cavadini, M.; Razzaque, S.; Finke, J. D.; Chiang, J.; Lott, B. Time delay of cascade radiation for TeV blazars and the measurement of the intergalactic magnetic field. The Astrophysical Journal Letters. 2011;733(2):L21. doi:10.1088/2041-8205/733/2/ L21

[66] Gamerman, D. Markov Chain Monte Carlo: Stochastic Simulation for Bayesian Inference.

[67] Lewis, A.; Bridle, S. Cosmological parameters from CMB and other data: a Monte Carlo approach. Physical Review D. 2002;66(10):103511. doi:10.1103/PhysRevD.66.103511 [68] MacKay, D. J. C. Information theory, inference and learning algorithms[M]. Cambridge

[69] Zhang, L.; Cheng, K. S.; Fan, J. H. The radio and gamma-ray luminosities of blazars. Publications of the Astronomical Society of Japan. 2001;53(2):207–213. doi:10.1093/pasj/53.2.207

[70] Narumoto, T.; Totani, T. Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution. Astrophysics and Space Science.2007;309(1–4):73–79. doi:10.1007/s10509-007-9453-4 [71] Abazajian, K. N.; Blanchet, S.; Harding, J. P. Contribution of blazars to the extragalactic diffuse gamma-ray background and their future spatial resolution. Physical Review D.

[72] Li, F.; Cao, X.-W. BL Lacertae objects and the extragalactic γ-ray background. Research in Astronomy and Astrophysics. 2011;11(8):879–887. doi:10.1088/1674-4527/11/8/001 [73] Finke, J. D.; Razzaque, S.; Dermer, C. D. Modeling the extragalactic background light from stars and dust. The Astrophysical Journal. 2010;712(1):238–249. doi:10.1088/0004-

[74] Domínguez, A.; Primack, J. R.; Rosario, D. J.; Prada, F.; Gilmore, R. C.; Faber, S. M.; et al. Extragalactic background light inferred from AEGIS galaxy-SED-type fractions. Monthly Notices of the Royal Astronomical Society. 2011;410(4):2556–2578. doi:10.1111/j.1365-

[75] Inoue, Y.; Inoue, S.; Kobayashi, M. A. R.; Makiya, R.; Niino, Y.; Totani, T. Extragalactic Background Light from hierarchical galaxy formation: gamma-ray attenuation up to the epoch of cosmic reionization and the first stars. The Astrophysical Journal. 2013;768

[76] Fan, Y. Z.; Dai, Z. G.; Wei, D. M. Strong GeV emission accompanying TeV blazar H1426 +428. Astronomy and Astrophysics. 2004;415:483–486. doi:10.1051/0004-6361:20034472

[77] Murase, K.; Takahashi, K.; Inoue, S.; Ichiki, K.; Nagataki, S. Probing intergalactic magnetic fields in the GLAST era through pair echo emission from TeV blazars. The Astro-

[78] Yang, C. Y.; Fang, J.; Lin, G. F.; Zhang, L.. Possible GeV Emission from TeV blazars. The

[79] Neronov, A.; Vovk, I. Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars. Science. 2010;328(5974):73. doi:10.1126/science.1184192

[80] Tavecchio, F.; Ghisellini, G.; Foschini, L.; Bonnoli, G.; Ghirlanda, G.; Coppi, P. The intergalactic magnetic field constrained by Fermi/large area telescope observations of the TeV

Chapman and Hall: London; 1997

106 New Insights on Gamma Rays

637X/712/1/238

2966.2010.17631.x

university press, 2003;P 640, ISBN-13: 978-0521642989

2011;84(10):103007. doi:10.1103/PhysRevD.84.103007

(2):197, 17 pp. doi:10.1088/0004-637X/768/2/197

physical Journal Letters. 2008;686(2): L67. doi:10.1086/592997

Astrophysical Journal. 2008;682(2):767–774. doi:10.1086/589326


**Recent Applications of Gamma Rays**
