1. Introduction

Gamma ray bursts are single, transient, short-lasting events (from a fraction of a second up to around a thousand seconds), and detected on the gamma ray sky. They are typically in the range between 10 keV and 20 MeV and are isotropically distributed on the celestial sphere, while they can occur at random directions, even a few times per day [1]. They are also often accompanied by an optical counterpart, late time X-ray signal, and the afterglow, which lasts for many days after the prompt phase and are detected in lower energies, down to the radio band.

The observed properties and energetics of gamma ray bursts has proven that at their hearts there is a cosmic explosion of an enormous power, which is definitely connected with the birth of compact stars. The newly born black hole is swallowing an extremely large amount of matter in a very short time. The accompanying process of ejection of rarefied and fast streams of plasma, which expand in the interstellar medium with a velocity close to the speed of light, is responsible for the gamma ray emission.

#### 1.1. History of GRBs observations

Gamma ray bursts have been first detected in the 1960s. The original measurement was by chance made by the US military service that operated the satellite Vela. The discovery was published in the Astrophysical Journal much later [2]. The authors of this work refer to the old hypothesis [3] that the supernova explosions should be accompanied by the gamma and X-ray emission. Nevertheless, for the confirmation of the supernova-GRB connection, astronomers had to wait more than 30 years more. Since 1970s, it was already known that GRBs are cosmic events, so that they have been studied by research satellites. The main break through was made in 1990s, when the BATSE satellite confirmed the isotropic distribution of GRBs on the sky. This was a strong argument for their extragalactic origin, and thus GRBs being one of the brightest sources of radiation in the universe. Another achievement of the BATSE mission was to establish two classes of bursts, which statistically cluster around the long (t > 2 s) and short (t < 2 s) events [4]. Here, the time t ¼ T<sup>90</sup> measures the 90% of counts detected by the counter.

Until 1997, there were no GRB counterparts found in the lower energy bands. For the first time, the Italian-Dutch satellite BeppoSAX detected the position of a GRB precisely enough, so that the localisation of an optical afterglow was possible for GRB970228 [5]. Here, the name of the GRB identifies the date of its observation, in the format rrmmdd. In May 1997, the first redshift of a GRB was estimated. The GRB970508, with 0:83 < z < 2:3 [6], confirmed that the events are observed from cosmological distances. Typically, the optical afterglow of a GRB is of 19th magnitude and can be observed from a couple weeks up to months after the burst. Its luminosity decrease with time has a power-law dependence. In addition, the X-ray afterglows can be observed, a few hours after the prompt phase. In some GRBs (like GRB991216), the emission lines have been reported in the Chandra, XMM-Newton, ASCA, and BeppoSAX data. Moreover, about half of the localized GRBs exhibits also the radio afterglows, seen about 1 day after the burst.

the mechanisms responsible for the ultimate production of electromagnetic transients called GRBs. We also speculate on the possible GRB-GW associacion scenarios. Finally, in the context of the recently discovered short GRB and its extended multiwalength emission, we present a model that connects the neutron-rich ejecta launched from the accreting torus in the GRB engine with the production of the unstable heavy isotopes produced in the so-called r-process. The radioactive decay of these isotopes is the source of additional emission observed in optical/infrared wavelengths and was confirmed to be found in a

Keywords: gamma ray burst: general, black hole physics, accretion, accretion disks, gravitational waves, neutrinos, nuclear reactions, nucleosynthesis, abundances

Gamma ray bursts are single, transient, short-lasting events (from a fraction of a second up to around a thousand seconds), and detected on the gamma ray sky. They are typically in the range between 10 keV and 20 MeV and are isotropically distributed on the celestial sphere, while they can occur at random directions, even a few times per day [1]. They are also often accompanied by an optical counterpart, late time X-ray signal, and the afterglow, which lasts for many days

The observed properties and energetics of gamma ray bursts has proven that at their hearts there is a cosmic explosion of an enormous power, which is definitely connected with the birth of compact stars. The newly born black hole is swallowing an extremely large amount of matter in a very short time. The accompanying process of ejection of rarefied and fast streams of plasma, which expand in the interstellar medium with a velocity close to the speed of light,

Gamma ray bursts have been first detected in the 1960s. The original measurement was by chance made by the US military service that operated the satellite Vela. The discovery was published in the Astrophysical Journal much later [2]. The authors of this work refer to the old hypothesis [3] that the supernova explosions should be accompanied by the gamma and X-ray emission. Nevertheless, for the confirmation of the supernova-GRB connection, astronomers had to wait more than 30 years more. Since 1970s, it was already known that GRBs are cosmic events, so that they have been studied by research satellites. The main break through was made in 1990s, when the BATSE satellite confirmed the isotropic distribution of GRBs on the sky. This was a strong argument for their extragalactic origin, and thus GRBs being one of the brightest sources of radiation in the universe. Another achievement of the BATSE mission was to establish two classes of bursts, which statistically cluster around the long (t > 2 s) and short (t < 2 s) events [4]. Here, the time t ¼ T<sup>90</sup> measures the 90% of counts detected by the counter. Until 1997, there were no GRB counterparts found in the lower energy bands. For the first time, the Italian-Dutch satellite BeppoSAX detected the position of a GRB precisely enough, so that

after the prompt phase and are detected in lower energies, down to the radio band.

number of sources.

is responsible for the gamma ray emission.

1.1. History of GRBs observations

1. Introduction

14 Cosmic Rays

The host galaxies of GRBs have been identified based on their precise localisations, thanks to Hubble Space Telescope (GRB970228, Sahu et al. [7]). For a long time, this was possible only in the case of long GRBs. It was found that their hosts are statistically bluer and more actively starforming and are also only moderately metal-rich, or even metal-poor [8, 9]. The redshifts measured for a sample of GRBs were clustered around z ¼ 1, and the redshift distribution was fitted well by the star formation rate dependence proposed by Porciani and Madau [10]. The local density of the GRBs derived from the luminosity function was about 0.44 Gpc<sup>3</sup> yr�1. In the case of short GRBs, their space distribution was found to be more 'local' than for the long ones, with V=Vmax ¼ 0:39, and 0.28, respectively, while the local density was found about 1.7 bursts per Gpc<sup>3</sup> per year [11]. These were just rough estimates, taking into account the fact that the short GRBs were missing the redshift data at that time. The number density of bursts should also be corrected by a beaming factor.

The important discovery which confirmed the origin of GRBs was the detection of emission lines, characteristic for supernova explosion, in the optical afterglows spectra (e.g., GRB030329). Hence, a very strong support was found for the idea of massive star's explosions being the progenitors of these events [12, 13]. The supernova connection was proposed earlier, since in some of the optical afterglow lightcurves the characteristic red bumps were detected, a few weeks after the GRB [14]. A new era in the GRB studies was opened with the launch of the Swift satellite in 2004. It found, for the first time, the afterglows of short GRBs. It occured, that contrary to long bursts, the short GRBs do not originate from the starburst galaxies neither the supernova explosions. A good candidate for their progenitor is a merger event, during which two compact objects collide, such as a binary neutron star meger. The Swift satellite detected GRBs at higher redshifts, e.g., GRB090423 at z ¼ 8:1 [15]. It occurred that a simple star formation law does not fit to the GRB distribution [16, 17]. It was found possible that the bursts are concentrating in the regions of specific value of metallicity.

In 2008, another high-energy mission was launched, Fermi. The detector GBM (gamma ray burst monitor) was placed onboard to detect gamma rays from cosmic transients. The burst GRB130427 was the most energetic event found to date and the energy of photons exceeded 90 GeV. The most recent achievements of the Fermi mission were connected with searching for high-energy electromagnetic counterparts to the gravitational wave events, discovered by LIGO interferometer.

#### 1.2. Models of GRBs origin

The gamma ray emission originates at rather large distances from the base of the jet. Therefore, the central engine driving the jets and forming its base are hidden from the observer and any studies of its structure must be grounded on the indirect analysis. The signal which would be emitted from the engine could be produced either in gravitational waves or in the neutrinos of MeV energies. Such neutrinos are rather impossible to be detected from cosmological distances. Much more promissing are neutrinos produced in the GRB jets which have energies in the order of GeV [18–20].

The constraints which are based on the observed isotropic equivalent energy of the bursts suggested that the total energy released during the explosion is in the order of the binding energy of a compact object with a stellar mass:

$$E = \frac{GM^2}{R} \approx 3 \times 10^{54} \text{ erg} \tag{1}$$

Among the models proposed to explain the short GRB population, the compact object merger model is most favored. Here, the duration of the event is limited by a much smaller size of the accretion disk, which forms after the remnant matter is left from the disrupted neutron star. The short bursts occur mostly in old, elliptical galaxies and within the regions of low star formation rate [29]. The most probable progenitor configuration is the NS-NS binary; the BH-NS was also studied, see. e.g., Narayan et al. [30]. Alternatively, also the magnetars, being extremely magnetized neutron stars when rotational energy is dissipated on the scale of

Gamma Ray Bursts: Progenitors, Accretion in the Central Engine, Jet Acceleration Mechanisms

http://dx.doi.org/10.5772/intechopen.76283

17

The accretion tori surrounding black holes are ubiquitous in the universe. They occupy centers of galaxies, or reside in binary systems composed of stellar mass black holes and main sequence stars, being a source of power for their ultraviolet or X-ray emission. In these kinds of objects, frequently the black hole accretion is accompanied with the ejection of jets, launched along the accretion disk axis. Such sources are then observed as the radio-loud quasars, driven by the action of supermassive black holes, or the 'microquasars', which are driven by the stellar mass black holes. The jets of plasma are accelerated up to the relativistic speeds, and

Similarly, in the case of ultrarelativistic jets that are sources of gamma rays in GRBs, the driving engine is supposedly the stellar mass black hole surrounded by an accretion disk. However, since the GRB events are transients that last only up to several hundred seconds, and not for thousands, or millions of years, the accretion process should not be persistent and last not too long. The limiting time of the GRB engine activity is governed by the amount of

From the computational point of view, the numerical model of any black hole accretion disk is based on standard equations of hydrodynamics (or MHD, if the magnetic fields are taken into account). The global parameters that enter the equations and act as scaling factors are the black

2.1. Chemical composition of the accretion disk in GRB engine and the equation of state

The temperature and density in the accretion disk feeding the gamma ray busts are governed by a huge accretion rate. The physical conditions make the disk undergo onset of nuclear reactions, since <sup>r</sup> � 1010 � <sup>10</sup>12g cm�<sup>3</sup> and <sup>T</sup> � <sup>10</sup><sup>9</sup> � 1011K. The disk is composed from the free protons, electrons and neutrons, and its electron fraction, defined as the ratio of protons to baryons, which is typically less than Ye ¼ 0:5. This is because of the neutronisation reactions, which are established by the condition of β-equilibrium, and greatly reduce the number density of free protons (balanced by electrons to satisfy the charge neutrality), in the cost of

seconds, may be able to produce Poynting dominated jets and power the GRBs [31].

2. Accretion onto a black hole as a driving engine of a GRB

emit the high-energy radiation, measured over the entire energy spectrum.

matter available for accretion, and by the rate of this process (Figure 1).

increasing the neutron number density.

hole mass, MBH, its angular momentum (called spin, a), and accretion rate, M\_ .

The burst durations are, however, much longer than the dynamical time, over which the matter can free fall onto such a star. The extended duration of the event must therefore be driven by a viscous process. The most plausible is the disk accretion process, which in addition provides a required collimation of the burst stream along the disk rotation axis. The appearance of a large amount of matter in the vicinity of a black hole, to be accreted with a few tens-hundreds of a second, implies an extremely violent process, most probably a birth of a new black hole.

The scenario of a compact object merger [21] was able to explain the energies required for a detection of the event from a cosmological distance [22]. It was thought first that this scenario could be universal for all the types of GRBs; however, the observations of the GRB host galaxies, their active star formation rates in some cases, and the discoveries of GRB-supernova connection led to a different scenario for the long bursts. The currently accepted scenario for the long GRB progenitor is the collapsar, or hypernova model [23, 24]. In this model, the massive iron core of a rapidly rotating, evolved massive star (typically, the Wolf-Rayet type of star) quickly collapses to form a black hole. Most of the stellar envelope is expelled, but the remaining part is slowed down by the backward shocks and fall back. The material which posseses large angular momentum is concentrating in the equatorial plane of the star and forms an accretion disk. The non-rotating matter is falling radially along the axis of rotation into the black hole and the empty funnel forms there, to help collimate the subsequenltly launched jets [25]. They have to break out the stellar envelope, and accelerate up to ultrarelativistic velocities, with Lorentz factors in the order of Γ � 100.

The hypernovae connected with long GRBs are a subgroup of the supernovae type I b/c (which do not exhibit neither hydrogen nor helium lines in their spectra) and constitute about 10% of this class [26]. Statistically, this should agree with the estimated rate of GRBs. Their occurrence rate is about 10�<sup>3</sup> of the supernova rate per galaxy per year [27]. The reason why not all the supernovae type I b/c (the core collapse supernovae) produce GRBs is most probably connected with the extremely low metallicities and rotation of the pre-SN star [28].

Among the models proposed to explain the short GRB population, the compact object merger model is most favored. Here, the duration of the event is limited by a much smaller size of the accretion disk, which forms after the remnant matter is left from the disrupted neutron star. The short bursts occur mostly in old, elliptical galaxies and within the regions of low star formation rate [29]. The most probable progenitor configuration is the NS-NS binary; the BH-NS was also studied, see. e.g., Narayan et al. [30]. Alternatively, also the magnetars, being extremely magnetized neutron stars when rotational energy is dissipated on the scale of seconds, may be able to produce Poynting dominated jets and power the GRBs [31].
