Testing hadronic models of gamma ray production at the core of Cen A

Abstract

Pierre Auger experiment has observed a few cosmic ray events above 55 EeV from the direction of the core of Cen A. These cosmic rays might have originated from the core of Cen A. High energy gamma ray emission has been observed by HESS from the radio core and inner kpc jets of Cen A. We are testing whether pure hadronic interactions of protons or heavy nuclei with the matter in the core region or photo-disintegration of heavy nuclei can explain the cosmic ray and high energy gamma ray observations from the core of Cen A. The scenario of p − γ interactions followed by photo-pion decay has been tested earlier by Sahu et al. (2012) and found to be consistent with the observational results. In this paper we have considered some other possibilities (i) the primary cosmic rays at the core of Cen A are protons and the high energy gamma rays are produced in p − p interactions,(ii) the primary cosmic rays are Fe nuclei and the high energy gamma rays are produced in F e − p interactions and (iii) the primary cosmic rays are Fe nuclei and they are photo-disintegrated at the core. The daughter nuclei de-excite and high energy gamma rays are produced. The high energy gamma ray fluxes expected in each of these cases are compared with the flux observed by HESS experiment to normalise the spectrum of the primary cosmic rays at the core. We have calculated the expected number of cosmic ray nucleon events between 55 EeV and 150 EeV in each of these cases to verify the consistencies of the different scenarios with the observations by Pierre Auger experiment. We find that if the primary cosmic rays are Fe nuclei then their photo-disintegration followed by de-excitation of daughter nuclei may explain the observed high energy particle emissions from the core of Cen A. The luminosity of the cosmic ray Fe nuclei required to explain the observational results of HESS and Pierre Auger is higher than the luminosity of the cosmic ray protons in the pγ interaction model. The required cosmic ray luminosity depends on the density of the low energy photons at the source which photo-disintegrate the Fe nuclei and the size of the emitting region.

Photo-disintegration of heavy nuclei at the core of Cen A

“…It was suggested earlier that the photo-disintegration of heavy nuclei at the core of Cen A could be the origin [4] of the gamma ray flux observed by HESS [2]. In this work we add the new gamma ray data from Fermi LAT [5] to the spectral energy distribution given in [1].…”

“…The wind medium at the core is moving relativistically with Lorentz factor Γ j . As discussed in [4] the gamma rays are detected only if the beaming of the gamma ray emission in the observer's frame is along the observer's line of sight. Depending on the angle of observation of the gamma ray emission θ ob , the Doppler factor of the wind medium is δ D = Γ −1 j (1 − β j cosθ ob ) −1 , where β j is the dimensionless speed of the wind rest frame with respect to the observer on earth.…”

“…Single zone synchrotron and synchrotron self-Compton (SSC) model have successfully explained the multiwavelenght data ranging from radio to gamma rays upto 1 GeV [1]. HESS data [2] observed in the energy window of few hundred of GeV to few TeV requires hadronic interaction to justify the emission [3,4]. More, recently a new component in the gamma ray spectrum from the core at GeV energy has been observed by Fermi LAT [5] which shows a hardening of gamma ray flux above 4 GeV to few tens of GeV.…”

“…The cosmic rays could be mostly protons or heavy nuclei [8]. The authors in [4] found that pure hadronic interactions between p-p and F e-p are not favored by the observational data from Pierre Auger Observatory [9,10] but photo-disintegration of Fe nuclei by local radiation field can successfully explain the cosmic ray and high energy γ ray emission from the core of Cen A. It can be an alternative to pγ interactions at the core discussed in [3].…”

“…It can be an alternative to pγ interactions at the core discussed in [3]. The scenario discussed in [4] assumes the cosmic rays are mostly Fe nuclei within the core of Cen A.…”