3. Star-forming regions in the Milky Way and the nonthermal emission

## 3.1. Cygnus X

Cygnus X is a part of the largest star-forming region of the constellation Cygnus in the northern galactic plane and has a distance of 1.4 kpc from the Earth.

There are many reasons why Cygnus X is an excellent region to investigate the origin of CRs:

• The emission is observable from radio to high-energy gamma-ray frequencies [28], at which in the energy range from GeV up to TeV, Cygnus X has the brightest emission in the northern hemisphere [29].

The modeling of the Cygnus region is based on the transport equation, Eq. (1). In order to solve the equation analytically, an emission from an isotropic and spatially homogeneous distributed part of the Cygnus region in its steady state is assumed. Additionally, a spatially independent diffusion of the particles within Cygnus X is applied. This assumption is reasonable since an extended region with a diameter of 77 pc will be considered, emission outside this zone is negligible, the region is very complex, and small inhomogeneities are expected to vanish at scales larger than the gyroradius. By this ansatz of the combination of a homogeneous injection and a leaky box, the variation of nð Þ r; γ; t only takes place at the scale of the radius of our region of interest. Using the quasi-neutrality of the plasma, the nonthermal radio and γ-ray emission can be correlated. This model has already been used for different starburst

Figure 4. Fermi's color map of Cygnus X (red cycle) distributed by Skyview HEASARC—HEALPixed by CDS; the map

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As no accurate data about the ambient conditions are known, the spectral index α, the target density Nt, and the magnetic field B are kept as best-fit parameters which lead to a coincidence

The continuous and catastrophic losses are essential for the transport within Cygnus X. Figure 6 compares all relevant losses in Cygnus X for electrons and protons separately.

Hereafter, the different γ-ray and neutrino fluxes, which are caused by different processes as explained in Figure 3, can be calculated separately according to Eq. (2) and by considering the related CR density distribution in Eq. (1). The agreement between γ-ray, synchrotron emission,

The left spectrum of Figure 7 displays the γ-ray flux considering leptonic as well as hadronic processes where the CR density has been derived for a homogeneous distribution in its steady

galaxies by [18], and details of the calculation can be found there.

was edited with Aladin v9.0. Additionally Cygnus X is pictured by a red cycle.

The variated parameters as a function of χ are shown in Figure 5.

and the model can be seen by the following spectra.

between nonthermal radio and γ-ray data and the prediction from the model.


All of these characteristics make Cygnus X a suitable natural laboratory for the astronomer to look beyond the usual constrained view.

The supernova remnant γ Cygni was first investigated using Fermi data, which provide information about the interstellar background by subtracting the radiation from γ Cygni. Cygnus X has a cocoon, in which freshly accelerated CRs are expected to be present, as its thermal emission exceeds 100 GeV. The SNR γ Cygni, which is in the cocoon, could cause the acceleration of protons even up to 80 300 TeV and electrons up to 6 30 TeV [35]. The accelerated particles could fill the whole cocoon if it is assumed that the main transport mechanism is diffusion [35]. Thus, γ Cygni has the potential to be the only accelerator in the cocoon. On the other hand, advection could dominate the transport mechanism, if an anisotropic emission of γ Cygni was observed [35]. Yet, there is still no final proof for this scenario. The cocoon could give hints about the transport mechanism and escape of CRs from their source.

<sup>1</sup> For example: γ Cygni J2021.0 + 4031e, which Milagro also detected at very high energies [30].

so, the solution just depends on the complexity of the source function and the continuous losses. Numerical approaches can treat the problem in a higher complexity; the analytical solutions have the advantage of being able to analyze the behavior of the source in more detail. In the following, we will present three localized regions, which are star-forming regions and

3. Star-forming regions in the Milky Way and the nonthermal emission

Cygnus X is a part of the largest star-forming region of the constellation Cygnus in the

There are many reasons why Cygnus X is an excellent region to investigate the origin of CRs: • The emission is observable from radio to high-energy gamma-ray frequencies [28], at which in the energy range from GeV up to TeV, Cygnus X has the brightest emission in

• Many potential accelerators such as supernova remnants,<sup>1</sup> pulsars, and pulsar wind nebulae can be found. The Milagro detections at > TeV energy point toward a possible hadronic emission scenario as leptonic processes cannot easily reach these energies, while they are natural for hadronic scenarios. IceCube has good visibility of the Cygnus region with muon neutrinos and could thus reveal a possible signal in neutrinos from that direction in the near future [31]. Many of these constituents are pictured in Figure 4. • Cygnus X contains a large number of H II regions [32], indicating a high level of ionization

All of these characteristics make Cygnus X a suitable natural laboratory for the astronomer to

The supernova remnant γ Cygni was first investigated using Fermi data, which provide information about the interstellar background by subtracting the radiation from γ Cygni. Cygnus X has a cocoon, in which freshly accelerated CRs are expected to be present, as its thermal emission exceeds 100 GeV. The SNR γ Cygni, which is in the cocoon, could cause the acceleration of protons even up to 80 300 TeV and electrons up to 6 30 TeV [35]. The accelerated particles could fill the whole cocoon if it is assumed that the main transport mechanism is diffusion [35]. Thus, γ Cygni has the potential to be the only accelerator in the cocoon. On the other hand, advection could dominate the transport mechanism, if an anisotropic emission of γ Cygni was observed [35]. Yet, there is still no final proof for this scenario. The cocoon could give hints about the transport mechanism and escape of CRs from their

For example: γ Cygni J2021.0 + 4031e, which Milagro also detected at very high energies [30].

northern galactic plane and has a distance of 1.4 kpc from the Earth.

deserve an adapted formulation of the CR density.

the northern hemisphere [29].

—possibly caused by CRs; see, e.g., [33, 34].

look beyond the usual constrained view.

3.1. Cygnus X

106 Cosmic Rays

source.

1

Figure 4. Fermi's color map of Cygnus X (red cycle) distributed by Skyview HEASARC—HEALPixed by CDS; the map was edited with Aladin v9.0. Additionally Cygnus X is pictured by a red cycle.

The modeling of the Cygnus region is based on the transport equation, Eq. (1). In order to solve the equation analytically, an emission from an isotropic and spatially homogeneous distributed part of the Cygnus region in its steady state is assumed. Additionally, a spatially independent diffusion of the particles within Cygnus X is applied. This assumption is reasonable since an extended region with a diameter of 77 pc will be considered, emission outside this zone is negligible, the region is very complex, and small inhomogeneities are expected to vanish at scales larger than the gyroradius. By this ansatz of the combination of a homogeneous injection and a leaky box, the variation of nð Þ r; γ; t only takes place at the scale of the radius of our region of interest. Using the quasi-neutrality of the plasma, the nonthermal radio and γ-ray emission can be correlated. This model has already been used for different starburst galaxies by [18], and details of the calculation can be found there.

As no accurate data about the ambient conditions are known, the spectral index α, the target density Nt, and the magnetic field B are kept as best-fit parameters which lead to a coincidence between nonthermal radio and γ-ray data and the prediction from the model.

The variated parameters as a function of χ are shown in Figure 5.

The continuous and catastrophic losses are essential for the transport within Cygnus X. Figure 6 compares all relevant losses in Cygnus X for electrons and protons separately.

Hereafter, the different γ-ray and neutrino fluxes, which are caused by different processes as explained in Figure 3, can be calculated separately according to Eq. (2) and by considering the related CR density distribution in Eq. (1). The agreement between γ-ray, synchrotron emission, and the model can be seen by the following spectra.

The left spectrum of Figure 7 displays the γ-ray flux considering leptonic as well as hadronic processes where the CR density has been derived for a homogeneous distribution in its steady

Figure 5. The total deviation χ<sup>2</sup> from the γ-ray and nonthermal radio data as a function of the magnetic field B and target density Nt. The best-fit parameters are represented by "+".

Figure 6. Continuous timescales (solid lines) and catastrophic timescales (dashed lines) for electrons (left) and protons (right) as a function of the Lorentz factor <sup>γ</sup> considering the best-fit parameters <sup>α</sup> <sup>¼</sup> <sup>2</sup>:4, Nt <sup>¼</sup> <sup>19</sup>:4 cm�3, and <sup>B</sup> <sup>¼</sup> <sup>9</sup> � <sup>10</sup>�<sup>6</sup> G. Here, <sup>τ</sup>fs <sup>¼</sup> <sup>R</sup>=<sup>c</sup> denotes the timescale of a free-streaming particle with the velocity of light c.

The influence of the models is evident as Tova et al. [36] suggest a neutrino spectrum from Cygnus X which is most likely not detectable, whereas the model from this work coincides with the limit of IceCube. Additionally, in Figure 8 the neutrino spectrum from Cygnus Cocoon is displayed, which has relatively a high flux. As IceCube has the highest sensitivity at 100 TeV and the spectral index of the predicted flux and the limit does not differ much from each other, a significant measurement by IceCube or IceCube-Gen2 may be soon possible. In fact, IceCube received already a 2 σ detection from Cygnus X. However, it is still not

Figure 8. Differential neutrino flux considering new parameters with and without (WO-D) diffusion. A neutrino flux from an alternative model calculated by [36] and IceCube's upper limit calculated for Cygnus X [31] is also displayed.

Figure 7. Synchrotron and γ-ray energy spectrum with and without (WO-D) consideration of diffusion; the source rate

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normalization factor q<sup>0</sup> was fitted on the observed gamma data.

Tova et al. (2017) investigated the γ-ray and neutrino spectrum from Cygnus X by assuming that the CR spectrum observed at Earth is also a representative for the Cygnus X. This contribution has been added by emission from the Cygnus Cocoon separately as well as from

significant.

state. Here, data from "Integral," "Fermi-LAT," and "ARGO-YBJ" as well as "Milagro" data, were considered. It is of great importance that data from "Integral" were taken into account, as the measurement at MeV energies strongly constraints the nonthermal bremsstrahlung and therefore the leptonic processes. Considering the synchrotron spectrum on the right side, a suitable diffusion coefficient helps to reach the γ-ray flux at 10 MeV and find the desired agreement.

The parameters from these spectra are used to find the neutrino spectrum which is displayed in Figure 8.

Figure 7. Synchrotron and γ-ray energy spectrum with and without (WO-D) consideration of diffusion; the source rate normalization factor q<sup>0</sup> was fitted on the observed gamma data.

Figure 8. Differential neutrino flux considering new parameters with and without (WO-D) diffusion. A neutrino flux from an alternative model calculated by [36] and IceCube's upper limit calculated for Cygnus X [31] is also displayed.

The influence of the models is evident as Tova et al. [36] suggest a neutrino spectrum from Cygnus X which is most likely not detectable, whereas the model from this work coincides with the limit of IceCube. Additionally, in Figure 8 the neutrino spectrum from Cygnus Cocoon is displayed, which has relatively a high flux. As IceCube has the highest sensitivity at 100 TeV and the spectral index of the predicted flux and the limit does not differ much from each other, a significant measurement by IceCube or IceCube-Gen2 may be soon possible. In fact, IceCube received already a 2 σ detection from Cygnus X. However, it is still not significant.

state. Here, data from "Integral," "Fermi-LAT," and "ARGO-YBJ" as well as "Milagro" data, were considered. It is of great importance that data from "Integral" were taken into account, as the measurement at MeV energies strongly constraints the nonthermal bremsstrahlung and therefore the leptonic processes. Considering the synchrotron spectrum on the right side, a suitable diffusion coefficient helps to reach the γ-ray flux at 10 MeV and find the desired

Figure 6. Continuous timescales (solid lines) and catastrophic timescales (dashed lines) for electrons (left) and protons (right) as a function of the Lorentz factor <sup>γ</sup> considering the best-fit parameters <sup>α</sup> <sup>¼</sup> <sup>2</sup>:4, Nt <sup>¼</sup> <sup>19</sup>:4 cm�3, and

<sup>B</sup> <sup>¼</sup> <sup>9</sup> � <sup>10</sup>�<sup>6</sup> G. Here, <sup>τ</sup>fs <sup>¼</sup> <sup>R</sup>=<sup>c</sup> denotes the timescale of a free-streaming particle with the velocity of light c.

Figure 5. The total deviation χ<sup>2</sup> from the γ-ray and nonthermal radio data as a function of the magnetic field B and target

density Nt. The best-fit parameters are represented by "+".

The parameters from these spectra are used to find the neutrino spectrum which is displayed

agreement.

108 Cosmic Rays

in Figure 8.

Tova et al. (2017) investigated the γ-ray and neutrino spectrum from Cygnus X by assuming that the CR spectrum observed at Earth is also a representative for the Cygnus X. This contribution has been added by emission from the Cygnus Cocoon separately as well as from identified and unidentified point sources. All calculations were carried out for 5 deg. � 5 deg. region which is subdivided in 0.25 deg. � 0.25 deg. In contrast to the multiwavelength model in this work, which considered the whole region as one source, and radio and hard γ-ray emission, [36] just calculated and considered γ-ray emission. Here, the validation is only a question of time and just depends on the measurement of IceCube.

## 3.2. Eta Carinae

η Carinae is a binary system which contains a massive LBV star and an O- or B-type companion star and is located 2.3 kpc from the Earth [37]. Astronomers have recognized it in the past in the 1840s and 1890s due to series of giant outbursts, which now formed into a nebula. The binary system itself is the most prominent source of one of the most active star-forming regions in the Milky Way.

In the astronomically near future, the system is expected to implode into a supernova. It has been already detected from hard X-rays to high-energy γ-rays. It was observed up to 300 GeV by Fermi-LAT during its full orbital period of 5.54 years [37] which is most commonly interpreted as a colliding wind binary. This binary creates a very strong terminal wind velocity of � <sup>500</sup> � 700 km s�<sup>1</sup> [38]. Thus, the acceleration might be performed at the shock fronts of the extensive wind collision. The interaction between the accelerated CRs and the stellar radiation fields, the magnetic fields, and the surrounding plasma leads to high-energy nonthermal emission due to nonthermal bremsstrahlung, inverse Compton emission, synchrotron radiation, and hadronic pion production [39]. This motivated the H.E.S.S. collaboration in 2012 to observe η Carinae between Fermi and H.E.S.S. energies as only then H.E.S.S. could measure this region of interest. The observations from 2014 to 2015 achieved a 13.6 σ pretrial measurement for the combined data set [40]. These observations indicate that η� Carinae could be another cosmic ray cradle in our galaxy.

searches for GeV neutrino sources with PINGU or ORCA. These have effective areas optimized to the GeV range, where atmospheric neutrino oscillations can be studied. At � 25 GeV, there is a minimum at which muon neutrinos from the atmosphere are strongly suppressed. This could open a small window in which the sensitivity for galactic GeV sources could be

Figure 9. Left figure: neutrino energy spectrum of η Carinae during the 500-day periastron passage from [42] is shown in green, the neutrino spectrum of the binary system η Carinae calculated by [41] in blue, the IceCube upper limit in gray,

. Right figure: neutrino spectrum of different Wolf-Rayet star binary

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A lepto-hadronic model to explain the nonthermal emission from Eta Carinae as an extended region has been presented in [42]. The spectral energy for both the periastron and apastron passage was derived. In this model, the part in the energy spectrum below 100 GeV is explained by inverse Compton emission. A prediction of a synchrotron spectrum is made in the radio to X-ray range. At the highest energies, E<sup>γ</sup> > 100 GeV, the spectrum is explained by a hadronic component, with a prediction of the detection potential for H.E.S.S. and CTA. Although the CRs have been described by just a simple power law without any cutoff, the calculation of the flux considered the loss timescale, the maximum energy, ambient conditions,

Deriving parameters from the comparison of the expected flux, observation data, and gammaneutrino correlation, the neutrino flux can be predicted. Figure 9 compares the prediction of the neutrino spectrum of η Carinae as derived from [41] (blue line) with the one in [42] (green

νμþνμ <sup>≃</sup> <sup>1</sup> � <sup>10</sup>�<sup>7</sup> GeV cm�<sup>2</sup> <sup>s</sup>�<sup>1</sup> for an <sup>E</sup>�<sup>2</sup> unbroken power law of IceCube considering the 7-year data [43] (gray line), as also indicated in Figure 9. There is a difference of nearly one order of magnitude which is rather unrealistic to measure. With Eta Carinae in the southern hemisphere, the region is quite difficult to see for IceCube. A future surface array in connection

Because of its location, KM3NeT has a better sensitivity to the southern hemisphere. For a 500 day exposure, the number of expected μ-neutrino events at KM3NeT for η Carinae between

�

, one obtains an upper flux limit

enhanced due to a strongly reduced background of atmospheric neutrinos [41].

Considering the position of η nebulae and the size of � 1

to an IceCube-Gen2 array could change this.

and the source geometry.

and the KM3NeT sensitivity for the whole region of 1�

line).

systems.

of E<sup>2</sup> Φ<sup>90</sup>%

Roughly, η Carinae can be sectioned into two phases:


A fully hadronic interpretation has been presented in [41]. In order to calculate the secondary emission from photons and neutrinos, it has been assumed that a fraction of η < 1 of the colliding wind luminosity goes into CRs. The wind luminosity is determined by the sum of the kinetic luminosities of the mass loss of the two stellar objects, so that LCR ≈ 1=2 � η� P <sup>i</sup>¼1, <sup>2</sup> <sup>M</sup>\_ <sup>i</sup> � <sup>V</sup><sup>2</sup> i � �, with Mi as the <sup>i</sup> <sup>¼</sup> 1st, 2nd companion, <sup>M</sup>\_ <sup>i</sup> as the mass loss, and Vi as the wind velocity of the two individual objects. The result of this calculation is shown in Figure 9. Here, the result for Eta Carinae is compared to other Wolf-Rayet star binary systems, which only have limits from Fermi observations. The neutrino fluxes are calculated by assuming a gamma-ray flux at the Fermi limit and are therefore also to be considered as upper limits to a possible neutrino flux from these heavy binary systems in the Milky Way. The expected emission of these point sources is at low energies for neutrino telescopes, which makes them difficult to detect with IceCube or KM3NeT. An interesting opportunity in the future could be

identified and unidentified point sources. All calculations were carried out for 5 deg. � 5 deg. region which is subdivided in 0.25 deg. � 0.25 deg. In contrast to the multiwavelength model in this work, which considered the whole region as one source, and radio and hard γ-ray emission, [36] just calculated and considered γ-ray emission. Here, the validation is only a

η Carinae is a binary system which contains a massive LBV star and an O- or B-type companion star and is located 2.3 kpc from the Earth [37]. Astronomers have recognized it in the past in the 1840s and 1890s due to series of giant outbursts, which now formed into a nebula. The binary system itself is the most prominent source of one of the most active star-forming regions

In the astronomically near future, the system is expected to implode into a supernova. It has been already detected from hard X-rays to high-energy γ-rays. It was observed up to 300 GeV by Fermi-LAT during its full orbital period of 5.54 years [37] which is most commonly interpreted as a colliding wind binary. This binary creates a very strong terminal wind velocity of � <sup>500</sup> � 700 km s�<sup>1</sup> [38]. Thus, the acceleration might be performed at the shock fronts of the extensive wind collision. The interaction between the accelerated CRs and the stellar radiation fields, the magnetic fields, and the surrounding plasma leads to high-energy nonthermal emission due to nonthermal bremsstrahlung, inverse Compton emission, synchrotron radiation, and hadronic pion production [39]. This motivated the H.E.S.S. collaboration in 2012 to observe η Carinae between Fermi and H.E.S.S. energies as only then H.E.S.S. could measure this region of interest. The observations from 2014 to 2015 achieved a 13.6 σ pretrial measurement for the combined data set [40]. These observations indicate that η� Carinae could be

1. Periastron passage: from the largest distance between both stars up to the shortest distance 2. Apastron passage: from the shortest distance between both stars up to the largest distance A fully hadronic interpretation has been presented in [41]. In order to calculate the secondary emission from photons and neutrinos, it has been assumed that a fraction of η < 1 of the colliding wind luminosity goes into CRs. The wind luminosity is determined by the sum of the kinetic luminosities of the mass loss of the two stellar objects, so that LCR ≈ 1=2 � η�

� �, with Mi as the <sup>i</sup> <sup>¼</sup> 1st, 2nd companion, <sup>M</sup>\_ <sup>i</sup> as the mass loss, and Vi as the wind velocity of the two individual objects. The result of this calculation is shown in Figure 9. Here, the result for Eta Carinae is compared to other Wolf-Rayet star binary systems, which only have limits from Fermi observations. The neutrino fluxes are calculated by assuming a gamma-ray flux at the Fermi limit and are therefore also to be considered as upper limits to a possible neutrino flux from these heavy binary systems in the Milky Way. The expected emission of these point sources is at low energies for neutrino telescopes, which makes them difficult to detect with IceCube or KM3NeT. An interesting opportunity in the future could be

question of time and just depends on the measurement of IceCube.

3.2. Eta Carinae

110 Cosmic Rays

in the Milky Way.

P

<sup>i</sup>¼1, <sup>2</sup> <sup>M</sup>\_ <sup>i</sup> � <sup>V</sup><sup>2</sup>

i

another cosmic ray cradle in our galaxy.

Roughly, η Carinae can be sectioned into two phases:

Figure 9. Left figure: neutrino energy spectrum of η Carinae during the 500-day periastron passage from [42] is shown in green, the neutrino spectrum of the binary system η Carinae calculated by [41] in blue, the IceCube upper limit in gray, and the KM3NeT sensitivity for the whole region of 1� . Right figure: neutrino spectrum of different Wolf-Rayet star binary systems.

searches for GeV neutrino sources with PINGU or ORCA. These have effective areas optimized to the GeV range, where atmospheric neutrino oscillations can be studied. At � 25 GeV, there is a minimum at which muon neutrinos from the atmosphere are strongly suppressed. This could open a small window in which the sensitivity for galactic GeV sources could be enhanced due to a strongly reduced background of atmospheric neutrinos [41].

A lepto-hadronic model to explain the nonthermal emission from Eta Carinae as an extended region has been presented in [42]. The spectral energy for both the periastron and apastron passage was derived. In this model, the part in the energy spectrum below 100 GeV is explained by inverse Compton emission. A prediction of a synchrotron spectrum is made in the radio to X-ray range. At the highest energies, E<sup>γ</sup> > 100 GeV, the spectrum is explained by a hadronic component, with a prediction of the detection potential for H.E.S.S. and CTA. Although the CRs have been described by just a simple power law without any cutoff, the calculation of the flux considered the loss timescale, the maximum energy, ambient conditions, and the source geometry.

Deriving parameters from the comparison of the expected flux, observation data, and gammaneutrino correlation, the neutrino flux can be predicted. Figure 9 compares the prediction of the neutrino spectrum of η Carinae as derived from [41] (blue line) with the one in [42] (green line).

Considering the position of η nebulae and the size of � 1 � , one obtains an upper flux limit of E<sup>2</sup> Φ<sup>90</sup>% νμþνμ <sup>≃</sup> <sup>1</sup> � <sup>10</sup>�<sup>7</sup> GeV cm�<sup>2</sup> <sup>s</sup>�<sup>1</sup> for an <sup>E</sup>�<sup>2</sup> unbroken power law of IceCube considering the 7-year data [43] (gray line), as also indicated in Figure 9. There is a difference of nearly one order of magnitude which is rather unrealistic to measure. With Eta Carinae in the southern hemisphere, the region is quite difficult to see for IceCube. A future surface array in connection to an IceCube-Gen2 array could change this.

Because of its location, KM3NeT has a better sensitivity to the southern hemisphere. For a 500 day exposure, the number of expected μ-neutrino events at KM3NeT for η Carinae between 10 TeV and 1 PeV amounts to 18 and for the diffuse astrophysical event to 0.03 [42], which could become interesting in the near future with a fully constructed KM3NeT detector.

synchrotron nature is present [48]. In contrast, a high-energy diffuse γ-ray emission contrib-

• Electrons are susceptible to severe synchrotron and inverse Compton (IC) losses. Assuming a formation of high-energy electrons close to the black hole and a magnetic field strength of <sup>100</sup> <sup>μ</sup> G, the synchrotron timescale is <sup>τ</sup>syn <sup>≈</sup> <sup>7</sup>:<sup>7</sup> � <sup>γ</sup>�<sup>1</sup> s, which corresponds to a mean free path

loss timescale yields <sup>τ</sup>io <sup>¼</sup> <sup>8</sup> � <sup>10</sup>�<sup>10</sup> � <sup>γ</sup> � Nt=5:<sup>7</sup> � 104 cm�<sup>3</sup> s. In contrast, the diffusion time-

• The loss timescale inside the dense molecular clouds at the Galactic Center is much shorter than the propagation timescale. Thus, the γ-ray emission contributed by electrons

• Therefore, a diffuse flux through the whole molecular zone demands an accelerator that boosts electrons up to ≥ 100 TeV [47] which requires unrealistic assumptions due to the

Thus, the calculation of the diffuse γ-ray flux to describe the emission detected by H.E.S.S. [47] requires only proton transport Eq. (1). By maintaining the radial dependency, the γ-ray luminosity as a function of the distance from the center can be calculated. The measurement of this quantity has already been presented at H.E.S.S. energies (E ≥ 1 TeV) by [47] and at Fermi (10 GeV ≤ E ≤ 0:3 TeV) energies by [49]. [50] also take the radial dependency of the continuous momentum loss rate due to hadronic pion production and the target density into account. This consideration complicates the transport Eq. (1) but gives an insight into the source distribution and reveals the real origin of the diffuse γ-ray emission. In doing so and by considering an isotropic distribution of the CR injection and gas, [50] present a hadronic one-component model (1CM), which considers only SgrA <sup>∗</sup> as the main source, and a two-component model (2CM) which considers SgrA <sup>∗</sup> as well as the SNR SgrA East. Figure 11 on the left side shows the calculated γ-ray emission from the hadronic pion production with the 1CM and 2CM. The filled blue circle is the measurement of Fermi-LAT PASS8 data [49] and the red filled triangles from H.E.S.S. [47]. The γ-ray fluxes in Figure 11 denoted by "Gaggero 1 & 2" are calculated by [49] who consider the CR large-scale population and a hard and conventional diffusion. However, the flux denoted by "Gaggero 3" works with a best-fit procedure and does not consider CR large-scale population. Each of the calculations is based on extensive transport equation which is only numerically solvable and takes the general radial dependency and the

The 2CM of [50] and the best-fit model of [49] describe the γ-ray data at higher energies sufficiently, though the best-fit model does not seem to consider a cutoff. Therefore, at higher energies, the discrepancies might become larger. The 2CM, which considers an exponential cutoff at 1 PeV, seems to describe Fermi-LAT as well as H.E.S.S. data best. However, only [50] tried to reproduce the γ-ray luminosity by their model. Due to the short distance of SgrA East to SgrA <sup>∗</sup> ( ≈ 2 pc), the radial distribution of the luminosity of both component models is quite

<sup>γ</sup><sup>1</sup>=<sup>3</sup> � ð Þ <sup>R</sup>=200pc

is expected to be focused around a small region around the black hole.

γcm where γ denotes the Lorentz factor. Moreover, the ionization

<sup>2</sup> � <sup>D</sup>0=4:<sup>7</sup> � <sup>1027</sup> �<sup>1</sup>

s.

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uted by electrons through the molecular zone is not realistic for various reasons:

of roughly <sup>λ</sup>syn <sup>≈</sup> <sup>3</sup>:<sup>9</sup> � 109

scale is given by <sup>τ</sup>diff <sup>¼</sup> <sup>2</sup>:<sup>1</sup> � 108

diffusion and magnetic field.

radial component of the diffusion tensor into account [49].

similar and presented in Figure 12.

## 3.3. Galactic Center

At a distance of 8.5 kpc from the Earth, the prominent supermassive black hole SgrA <sup>∗</sup> represents the center of our galaxy. This central region in the galaxy is peculiar, as it reveals a high molecular density but no strong enhancement in star formation—a phenomenon that is also observed for other centers of galaxies [44]. Thus, it can formally not be considered as a star-forming region. Nevertheless, this region shows enhanced nonthermal emission at a broad energy range, which makes it equally interesting as those star-forming regions in the galaxy that we discuss here. This is linked with the ambient conditions but as well as with the sources within the region. SgrA <sup>∗</sup> is surrounded by a circumnuclear disk (CND) with R≃3 pc and a total mass of 10<sup>6</sup> M<sup>⊙</sup> [45]. SgrA West the "minispiral," respectively, is a thermal radio source with three spiral arms of molecular gas that surrounds SgrA <sup>∗</sup>. In every respect, the Galactic Center is very crowded especially by the stellar population as well as by molecular, atomic, and ionized gas. These characteristics make it rich on red giant stars, massive stars, and hundreds of OB and Wolf-Rayet stars [46]. Some prominent SNRs can be found near the Galactic Center, among those the prominent SNR SgrA East. This SNR is just 2.5 pc far away from the Center, has an elliptical shell along the galactic plane, and is surrounded by ionized gas. A visualization is exemplified in Figure 10.

SgrA <sup>∗</sup> is supposed to generate high-energy CRs [47]. In fact, a diffuse γ-ray up to several tens TeV has been observed by H.E.S.S. within a radius of approximately 200 pc around the Center. Considering just a radius of about 8 pc, a large-scale diffuse radio emission, the radio halo, of a

Figure 10. Inner 10 pc Galactic Center at 5 GHz.

synchrotron nature is present [48]. In contrast, a high-energy diffuse γ-ray emission contributed by electrons through the molecular zone is not realistic for various reasons:

10 TeV and 1 PeV amounts to 18 and for the diffuse astrophysical event to 0.03 [42], which

At a distance of 8.5 kpc from the Earth, the prominent supermassive black hole SgrA <sup>∗</sup> represents the center of our galaxy. This central region in the galaxy is peculiar, as it reveals a high molecular density but no strong enhancement in star formation—a phenomenon that is also observed for other centers of galaxies [44]. Thus, it can formally not be considered as a star-forming region. Nevertheless, this region shows enhanced nonthermal emission at a broad energy range, which makes it equally interesting as those star-forming regions in the galaxy that we discuss here. This is linked with the ambient conditions but as well as with the sources within the region. SgrA <sup>∗</sup> is surrounded by a circumnuclear disk (CND) with R≃3 pc and a

with three spiral arms of molecular gas that surrounds SgrA <sup>∗</sup>. In every respect, the Galactic Center is very crowded especially by the stellar population as well as by molecular, atomic, and ionized gas. These characteristics make it rich on red giant stars, massive stars, and hundreds of OB and Wolf-Rayet stars [46]. Some prominent SNRs can be found near the Galactic Center, among those the prominent SNR SgrA East. This SNR is just 2.5 pc far away from the Center, has an elliptical shell along the galactic plane, and is surrounded by ionized

SgrA <sup>∗</sup> is supposed to generate high-energy CRs [47]. In fact, a diffuse γ-ray up to several tens TeV has been observed by H.E.S.S. within a radius of approximately 200 pc around the Center. Considering just a radius of about 8 pc, a large-scale diffuse radio emission, the radio halo, of a

M<sup>⊙</sup> [45]. SgrA West the "minispiral," respectively, is a thermal radio source

could become interesting in the near future with a fully constructed KM3NeT detector.

3.3. Galactic Center

112 Cosmic Rays

total mass of 10<sup>6</sup>

gas. A visualization is exemplified in Figure 10.

Figure 10. Inner 10 pc Galactic Center at 5 GHz.


Thus, the calculation of the diffuse γ-ray flux to describe the emission detected by H.E.S.S. [47] requires only proton transport Eq. (1). By maintaining the radial dependency, the γ-ray luminosity as a function of the distance from the center can be calculated. The measurement of this quantity has already been presented at H.E.S.S. energies (E ≥ 1 TeV) by [47] and at Fermi (10 GeV ≤ E ≤ 0:3 TeV) energies by [49]. [50] also take the radial dependency of the continuous momentum loss rate due to hadronic pion production and the target density into account. This consideration complicates the transport Eq. (1) but gives an insight into the source distribution and reveals the real origin of the diffuse γ-ray emission. In doing so and by considering an isotropic distribution of the CR injection and gas, [50] present a hadronic one-component model (1CM), which considers only SgrA <sup>∗</sup> as the main source, and a two-component model (2CM) which considers SgrA <sup>∗</sup> as well as the SNR SgrA East. Figure 11 on the left side shows the calculated γ-ray emission from the hadronic pion production with the 1CM and 2CM. The filled blue circle is the measurement of Fermi-LAT PASS8 data [49] and the red filled triangles from H.E.S.S. [47]. The γ-ray fluxes in Figure 11 denoted by "Gaggero 1 & 2" are calculated by [49] who consider the CR large-scale population and a hard and conventional diffusion. However, the flux denoted by "Gaggero 3" works with a best-fit procedure and does not consider CR large-scale population. Each of the calculations is based on extensive transport equation which is only numerically solvable and takes the general radial dependency and the radial component of the diffusion tensor into account [49].

The 2CM of [50] and the best-fit model of [49] describe the γ-ray data at higher energies sufficiently, though the best-fit model does not seem to consider a cutoff. Therefore, at higher energies, the discrepancies might become larger. The 2CM, which considers an exponential cutoff at 1 PeV, seems to describe Fermi-LAT as well as H.E.S.S. data best. However, only [50] tried to reproduce the γ-ray luminosity by their model. Due to the short distance of SgrA East to SgrA <sup>∗</sup> ( ≈ 2 pc), the radial distribution of the luminosity of both component models is quite similar and presented in Figure 12.

Although the radial distribution of H.E.S.S. is reproduced pretty well, the Fermi distribution exhibits discrepancies. This may be due to the simplification of the model which assumed an isotropic distribution around the Galactic Center or due to additional sources which are not identified or studied yet. The comparison between the expected and the measured luminosity does not suggest an isotropic injection of protons from one centrally located accelerator into the Central Molecular Zone as if it does it would not be able to suffice the observed luminosity. In contrast an additional source or sources could change the distribution in the right way.

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http://dx.doi.org/10.5772/intechopen.75429

Figure 13 also displays the IceCube's upper flux adapted for the region of interest as well as the sensitivity of KM3NeT. Again, due to atmospheric μ, the capability of KM3NeT is higher to detect neutrinos from the southern hemisphere and thus from the Galactic Center. Both observatories are still not able to detect any neutrinos from the direction of the Galactic Center.

Star-forming regions are of prime importance for investigating the origin of CRs, as they can be considered as CR birthplaces. During the past decades, a large variety of telescopes went into operation, starting to shed light on the nonthermal multi-messenger picture from star-forming regions in the Milky Way. Of particular interest are three concrete regions in the Milky Way: 1. The Cygnus complex is the most interesting star-forming region in the northern sky. It reveals nonthermal emission from diffuse regions and point sources up to > TeV energy, in particular detected by the Milagro detector. This makes it a prime candidate to search for hadronic interaction signatures. The northern location is beneficial for the IceCube experiment, which can detect neutrinos in the TeV range with a spatial resolution of below 1<sup>∘</sup> from the northern sky. The neutrino flux from Cygnus X approaches the IceCube's upper limit and past already the 2 σ deviations from background could be detected in the past. Model predictions show that a detection of the region could already be possible with IceCube, certainly with

the next-generation array IceCube-Gen2 for a detection threshold in the TeV range.

However, the model reproduces the γ-ray flux and the luminosity satisfactorily.

Figure 13. Neutrino spectrum from 1CM (left) and from 2CM (right) for Ecut ¼ 1 PeV.

4. Summary and outlook

Accordingly, the neutrino spectrum is derived for the 1CM as well as for the 2CM.

Figure 11. γ-ray emission from the inner 70 pc of the Galactic Center by [50] and by [49].

Figure 12. <sup>γ</sup>-ray luminosity as function of the distance from SgrA <sup>∗</sup> for <sup>E</sup>cut <sup>¼</sup> 1 PeV.

Figure 13. Neutrino spectrum from 1CM (left) and from 2CM (right) for Ecut ¼ 1 PeV.

Although the radial distribution of H.E.S.S. is reproduced pretty well, the Fermi distribution exhibits discrepancies. This may be due to the simplification of the model which assumed an isotropic distribution around the Galactic Center or due to additional sources which are not identified or studied yet. The comparison between the expected and the measured luminosity does not suggest an isotropic injection of protons from one centrally located accelerator into the Central Molecular Zone as if it does it would not be able to suffice the observed luminosity. In contrast an additional source or sources could change the distribution in the right way. However, the model reproduces the γ-ray flux and the luminosity satisfactorily.

Accordingly, the neutrino spectrum is derived for the 1CM as well as for the 2CM.

Figure 13 also displays the IceCube's upper flux adapted for the region of interest as well as the sensitivity of KM3NeT. Again, due to atmospheric μ, the capability of KM3NeT is higher to detect neutrinos from the southern hemisphere and thus from the Galactic Center. Both observatories are still not able to detect any neutrinos from the direction of the Galactic Center.
