PeV Gamma Rays from Interactions of Ultra High Energy Cosmic Rays in the Milky Way

The observational data from extensive air shower experiments like Hires [1], AGASA [2], Yakutsk [3] and Auger [4] have been widely used to undestand the origin of ultra high energy cosmic rays (UHECRs) [5,6,7]. The maximum energy of cosmic rays E max from a cosmic accelerator is determined by the size (R), magnetic field (B) of the acceleration region, shock velocity (β) and charge of the cosmic ray particle (Z), E max = ZeβBR [8]. It is not yet known what could be the maximum energy of the cosmic rays originated from sources inside Milky Way. Most of the cosmic ray sources in the Milky Way are located near its center, as a result a galactocentric anisotropy is expected in the UHECR spectrum. The observed UHECR spectrum does not show such anisotropy upto 1EeV energy. The properties of cosmic rays in the energy range of 0.1PeV and 1EeV have been studied by KASCADE-Grande experiment [9]. It has been shown earlier that the knee like structure near 4 PeV in the all particle spectrum is due to the decrease in the flux of light nuclei [10,11]. The all particle energy spectrum of cosmic rays reconstructed from the KASCADE-Grande data using three different techniques and the hadronic interaction models QGSJET II, FLUKA shows concavity and a small discontinuity near 0.01 EeV, 0.1 EeV respectively [12]. The energy dependence of composition of UHECRs can not be determined directly as their flux decreases with increasing energy. It is determined indirectly by observing extensive air showers in the atmosphere. The average depth of the shower maximum depends on the energy and mass of the primary particle. The current observational results tell us above 1EeV there is a gradual increase in average mass of cosmic rays upto energy 59EeV [13]. Recently it has been discussed [14] that cosmic ray nuclei of energy 1EeV to 10EeV can be originated from old gamma ray bursts or supernova explosions in our Galaxy and their directions may become isotropic as they spend long time in the micro-Gauss magnetic field of Galaxy. The inhomogeneous magnetic field in our Galaxy is responsible for trapping the charged particles for long time. The field strength has been estimated to be between 60µG and 400µG in the inner region of the Galaxy [15]. Inside Milky Way there are supernova remnants (SNRs), pulsar wind nebulae (PWN), microquasars and many unidentified sources of γ rays. The TeV gamma ray spectrum from some of the SNRs in our Galaxy may be explained with hadronic interaction models and inverse Compton emission by electrons [16,17]. SNRs have long been identified as attractive sites for cosmic ray accelerations. In particular, near the Galactic center (GC) region, there is a large population of γ ray sources. The high energy photon fluxes emitted by them have been observed by HESS [18], FERMI [19], AGILE [20], MILAGRO [21] and other gamma ray detectors. The photon flux from GC region as observed by FERMI-LAT has been fitted with conventional theoretical model of cosmic ray interactions with ambient gas medium, inverse Compton scattering of electrons and positrons by interstellar radiations [22]. If one considers only the protons, then pp and pγ interactions are the possible mechanisms of γ ray production. The crosssection of pp interactions is much higher that pγ interactions [23]. In the GC region the radiation and interstellar gas densities are high compared to the outer regions of Galaxy. The Galactic interstellar radiation field has been calculated by Moskalenko et al. [24]. With the average number density of hydrogen gas molecules n = 10cm -3 and 120cm -3 [15] and photon density of Galactic radiation field [24], one finds pp process is much more important than pγ process in the central region of Milky Way. In these interactions nearly 20% of the original cosmic ray proton's energy goes to the neutral pion and it decays to two high energy gamma rays of equal energy. Hence, we expect 100 PeV gamma rays from 1EeV cosmic ray protons. 1 EeV proton of extra-galactic origin may appear like UHECR of Galactic origin in the high magnetic field of GC region. If the UHECRs are protons and they are of Galactic origin then a galactocentric flux of secondary PeV gamma rays may be expected in interactions of these protons with ambient matter. The UHECR heavy nuclei would be deflected more in Galactic magnetic fields compared to the protons. They would interact with ambient hydrogen molecules producing pions, subsequently decaying to very high energy photons and neutrinos. Also there may be photo-disintegration of heavy nuclei and photo-pion production by heavy nuclei. In this paper we calculate the secondary PeV-EeV gamma ray flux produced by the interactions of diffuse UHECRs with ambient matter and radiations in the central region of Milky Way. The secondary PeV gamma rays/neutrinos may be anisotropic because of the strong magnetic field in the GC region. Moreover, if the UHECRs are produced by point sources in the central region of Milky Way th

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