TeV gamma rays from blazars beyond z=1?

TL;DR

This paper proposes that TeV gamma rays observed from blazars beyond redshift 1 could be secondary photons produced by high-energy protons, challenging the standard gamma-ray horizon and implying very weak extragalactic magnetic fields.

Contribution

It introduces a novel mechanism where high-energy protons produce secondary gamma rays, allowing TeV observations from distant blazars beyond the conventional horizon.

Findings

TeV gamma rays from z>1 can be explained by secondary photon production.

Extragalactic magnetic fields are constrained to very weak levels (0.01-10 fG).

Protons with energies >0.1 EeV can be effectively accelerated in AGN jets.

Abstract

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 z>1, if the observed gamma rays are secondary photons produced in interactions of high-energy protons originating from the blazar jet and propagating over cosmological 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>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 0.01 < fG < 10 fG, assuming random fields with a correlation length of 1 Mpc, and that acceleration of E> 0.1 EeV protons in the jets of active galactic nuclei can be very effective.