Fermi-LAT gamma-ray anisotropy and intensity explained by unresolved Radio-Loud Active Galactic Nuclei

TL;DR

This paper models various subclasses of radio-loud AGN to explain the gamma-ray background's intensity and anisotropy, showing they can account for all observed properties and predicting future anisotropy measurements.

Contribution

It provides updated gamma-ray luminosity functions for AGN subclasses and demonstrates their combined ability to explain the IGRB's intensity and anisotropy.

Findings

Radio-loud AGN can explain the entire IGRB intensity and anisotropy.

Misaligned AGN dominate the intensity but not anisotropy.

High-synchrotron-peaked BL Lacs dominate the anisotropy.

Abstract

Radio-loud active galactic nuclei (AGN) are expected to contribute substantially to both the intensity and anisotropy of the isotropic gamma-ray background (IGRB). In turn, the measured properties of the IGRB can be used to constrain the characteristics of proposed contributing source classes. We consider individual subclasses of radio-loud AGN, including low-, intermediate-, and high-synchrotron-peaked BL Lacertae objects, flat-spectrum radio quasars, and misaligned AGN. Using updated models of the gamma-ray luminosity functions of these populations, we evaluate the energy-dependent contribution of each source class to the intensity and anisotropy of the IGRB. We find that collectively radio-loud AGN can account for the entirety of the IGRB intensity and anisotropy as measured by the Fermi Large Area Telescope (LAT). Misaligned AGN provide the bulk of the measured intensity but a negligible contribution to the anisotropy, while high-synchrotron-peaked BL Lacertae objects provide the dominant contribution to the anisotropy. In anticipation of upcoming measurements with the Fermi-LAT and the forthcoming Cherenkov Telescope Array, we predict the anisotropy in the broader energy range that will be accessible to future observations.