Observable spectral and angular distributions of $gamma$-rays from extragalactic ultrahigh energy cosmic ray accelerators: the case of extreme TeV blazars. (arXiv:1912.10570v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Khalikov_E/0/1/0/all/0/1">E.V. Khalikov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dzhatdoev_T/0/1/0/all/0/1">T.A. Dzhatdoev</a>
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
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
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