TeV gamma rays from blazars beyond z = 1?

At TeV energies, 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 call for a revision of the standard paradigm. While robust observational evidence for TeV sources at redshifts z 1 is lacking at present, the growing number of TeV blazars with redshifts as large as z -- 0.5 suggests the possibility that the standard blazar models may have to be reconsidered. We show that TeV gamma rays can be observed even from a source at zï 1, if the observed gamma rays are secondary photons produced in interactions of high-energy protons originating from the blazar jet and propagating over cosmologica! distances almost rectilinearly. This mechanism was initially proposed as a possible explanation for the TeV gamma rays observed from blazars with redshifts z ~ 0.2, for which some other explanations were possible. For TeV gamma-ray radiation detected from a blazar with z S 1, this model would provide the only viable interpretation consistent with conventional physics. It would also have far-reaching astronomical and cosmological ramifications. In particular, this interpretation would imply that extragalactic magnetic fields along the line of sight are very weak, in the range 10~17 G < B < 10"14 G, assuming random fields with a correlation length of 1 Mpc, and that acceleration of E > 1017 eV protons in the jets of active galactic nuclei can be very effective. [ABSTRACT FROM AUTHOR]