Felix Aharonian, Warren Essey, Alexander Kusenko, Anton Prosekin
At very high energies (VHE), the gamma-ray horizon of the universe is limited to redshifts z<<1, and, therefore, any observation of TeV radiation from a source located beyond z=1 would require a dramatic revision of the standard scenarios of propagation of VHE photons through intergalactic radiation and magnetic fields. This appears to be the case for the TeV blazar PKS 0447-439, for which a redshift z>1.246 was recently reported. In this paper we argue that the reported large redshift can be compatible with gamma-ray emission extending to TeV energies, without invoking exotic new physics, if one assumes that the observed gamma rays are secondary photons produced in interactions of high-energy protons originating from the blazar jet and propagating over the cosmological distances almost rectilinearly. This hypothesis was initially proposed as a possible explanation for the TeV gamma rays observed from blazars with relatively large, yet modest redshifts, z~0.2, for which other explanations were possible. In the case of PKS 0447-439, it provides the only viable interpretation of the VHE signal consistent with conventional physics. If the observability of TeV gamma rays from blazars at z>1 is confirmed by future observations, our interpretation will have far-reaching ramifications for gamma-ray astronomy. Furthermore, this interpretation implies that intergalactic magnetic fields (IGMFs) along the line of sight are very weak, in the range 0.01 fG < B < 1 fG. and that acceleration of E> 0.1 EeV protons in the AGN jets is indeed very efficient.
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http://arxiv.org/abs/1206.6715
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