http://arxiv.org/abs/1912.10570
Context. Ultrahigh energy protons and nuclei from extragalactic cosmic ray sources produce secondary $\gamma$-rays, electrons and positrons on cosmic microwave background (CMB) and extragalactic background light (EBL) photons. These secondary particles initiate intergalactic electromagnetic (EM) cascades, eventually resulting in observable fluxes of $\gamma$-rays in the GeV-TeV energy domain. The total spectrum of such cascade $\gamma$-rays of hadronic nature is significantly harder than the one usually expected from blazars. The spectra of some sources known as “extreme TeV blazars” were claimed to be well-described by this “intergalactic hadronic cascade model”. Aims. We calculate the shape of the observable spectrum inside the point spread function (PSF) of a typical imaging atmospheric Cherenkov telescope (IACT), as well as the observable angular distibution of $\gamma$-rays, for the first time taking into account the effect of primary proton deflection in a realistic extragalactic magnetic field (EGMF). We investigate if these deflections significantly change the shape of the observable spectrum and whether the observable angular distibution could be detected with currently operating or projected $\gamma$-ray telescopes. Methods. We estimate the width of the observable $\gamma$-ray angular distribution from simple geometrical considerations. In addition, we employ a hybrid code previously delevoped by us, supplementing it with new routines to compute the observable angle for detectable $\gamma$-rays. Results. The observable point-like spectrum at milti-TeV energies is much softer than the one averaged over all values of the observable angle. The observable angular distribution is found to be sufficiently broad to enable a robust identification of the extended emission around the sources with next-generation $\gamma$-ray telescopes. \abridged
E. Khalikov and T. Dzhatdoev
Tue, 24 Dec 19
60/79
Comments: 13 pages, 14 figures
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