The isotropic $\gamma$-ray emission above 100 GeV: where do very high energy $\gamma$ rays come from?

TL;DR

This study investigates the origins of very high energy gamma-ray emission above 100 GeV, finding that blazars are the dominant known sources but most VHE photons still have unknown origins, highlighting the need for further research.

Contribution

The paper provides the first detailed analysis of Fermi-LAT data above 100 GeV at high Galactic latitudes, quantifying the contribution of blazars and revealing that most VHE gamma-ray photons originate from unknown sources.

Findings

Blazars account for approximately 23% of extragalactic VHE gamma-ray photons.

Over 77% of VHE photons have unknown origins.

Blazars are the dominant known source class above 100 GeV.

Abstract

Astrophysical sources of very high energy (VHE; $>100$ GeV) $\gamma$ rays are rare, since GeV and TeV photons can be only emitted in extreme circumstances involving interactions of relativistic particles with local radiation and magnetic fields. In the context of the Fermi Large Area Telescope (LAT), only a few sources are known to be VHE emitters, where the largest fraction belongs to the rarest class of active galactic nuclei: the blazars. In this work, we explore Fermi-LAT data for energies $>100$ GeV and Galactic latitudes $b > |50^{\circ}|$ in order to probe the origin of the extragalactic isotropic $\gamma$-ray emission. Since the production of such VHE photons requires very specific astrophysical conditions, we would expect that the majority of the VHE photons from the isotropic $\gamma$-ray emission originate from blazars or other extreme objects like star-forming galaxies, $\gamma$-ray bursts, and radio galaxies, and that the detection of a single VHE photon at the adopted Galactic latitudes would be enough to unambiguously trace the presence of such a counterpart. Our results suggest that blazars are, by far, the dominant class of source above 100 GeV, although they account for only $22.8^{+4.5}_{-4.1}\%$ of the extragalactic VHE photons. The remaining $77^{+4.1}_{-4.5}\%$ of the VHE photons still have an unknown origin.