TeV gamma rays from blazars beyondz=1?

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 10 −17 G < B < 10 −14 G, assuming random fields with a correlation length of 1 Mpc, and that acceleration of E ≥ 10 17 eV protons in the jets of active galactic nuclei can be very effective.

Understanding the spectrum of a distant blazar PKS 1424+240 and its implications

“…For a non-spherical accretion with a jet, the Eddington luminosity is not necessarily a limiting factor, and there is growing evidence of superEddington luminosities in relativistic outflows in GRBs and in very powerful blazars [33]. The required luminosity is consistent with general principles of acceleration, as long as most of the accretion energy is converted efficiently to the kinetic energy of the jet, rather than to thermal radiation of the accretion flow [8].…”

“…While at present PKS 1424+240 is the most EBL-obscured known source, one expects to detect additional TeV sources at redshifts z ∼ 1 [8], especially with the advent of the Cherenkov Telescope Array (CTA) capable of an unbiased allsky survey [13]. A combined analysis of spectral and temporal information, similar to our analysis of PKS 1424+240, will be a powerful probe of EBL, when applied to the large dataset expected in the near future [8,13]. Fig.…”

“…Only the lower bound exists on the redshift of PKS 1424+240. Secondary gamma rays of TeV energies can be seen from sources beyond z = 1 [8]. We will, therefore, consider two different redshifts consistent with the lower bound.…”

“…At the higher end of the spectrum, gamma rays must interact with extragalactic background light (EBL) and lose energy through pair production. However, line-of-sight interactions of cosmic rays with cosmic microwave background radiation (CMBR) and EBL can generate secondary gamma rays relatively close to the observer, which provides a plausible explanation of the observed hard spectra [1][2][3][4][5][6][7][8][9][10][11][12][13]. Primary gamma rays dominate the observed radiation from nearby blazars, and only the more distant sources provide an opportunity to study secondary gamma rays.…”

Gamma rays from blazars

“…An interesting scenario interprets these peculiarities invoking the existence of a beam of ultra-high energy hadrons escaping from the jet and producing the γ-ray emission on the way toward the Earth through Bethe-Heitler and photomeson cooling [95]. A robust and testable prediction of this model is the presence of a detectable hard γ-ray tail extending above 10 TeV [96] even for sources located at redshift close to 1 [97]. In the standard models the EBL absorption would prevent the detection of photons with these extreme energies.…”