The origin of gamma-ray radiation of extreme TeV blazars (ETBs) — active galactic nuclei with unusually hard observable spectra in the TeV energy region — is still poorly understood. This is especially unfortunate since ETB studies are important in extragalactic background light measurements, and extremely important in extragalactic magnetic field measurements and axion-like particle searches. Besides very hard gamma-ray spectra, ETBs are usually characterized by relatively weak (~dozens of percent) and slow (typical timescale of ~months-years) variability of the gamma-ray flux. These unusual properties of ETBs could in principle be explained if the observable gamma-rays were formed not inside these sources, but rather in the intergalactic medium as a result of the development of electromagnetic cascades. A scenario assuming the primary ultrahigh energy (E>1 EeV) protons — the so-called intergalactic hadronic cascade scenario (IHCM) — enjoyed especially high popularity during the last decade. In the present work we calculate the observable spectrum and angular distribution of observable gamma-rays from extreme TeV blazars in the framework of the IHCM scenario, for the first time taking into account the effect of primary proton deflection in filaments and galaxy clusters of a realistic extragalactic magnetic field. We show that the observable point-like spectrum (i.e. the spectrum inside the point spread function of a typical imaging atmospheric Cherenkov telescope) at multi-TeV energies is significantly softer than the one averaged over all values of the observable angle. The width of the observable angular distribution at E>10 GeV is greater than or comparable to the extent of the point spread function of operating and next-generation gamma-ray telescopes. These results imply that electromagnetic cascades from primary ultrahigh energy protons do not enhance the effective transparency of the Universe in the very high energy range significantly. The reported study was funded by RFBR, Russia, project number 20-32-70169.
|Reference to paper (DOI or arXiv)
|doi: 10.1093/mnras/stab1393 arxiv: https://arxiv.org/abs/1912.10570