Misc,
TeV gamma rays from blazars beyond z=1?
(2012)cite arxiv:1206.6715Comment: 10 pages, 3 figures.
Abstract
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.
