Source-count Distribution of Gamma-Ray Blazars

TL;DR

This study uses ten years of Fermi-LAT data to derive the intrinsic source-count distribution of extragalactic gamma-ray blazars, revealing their flux distribution, evolution, and contribution to the gamma-ray background.

Contribution

It provides the first detailed measurement of the blazar logN-logS distribution down to very low fluxes and constrains their evolutionary models and contribution to the gamma-ray background.

Findings

The logN-logS distribution flattens at low fluxes.

Pure Density Evolution model best fits the data.

Blazars contribute approximately 50% of the extragalactic gamma-ray background.

Abstract

With ten years of operation and an exceptional dataset, the Fermi-Large Area Telescope allows us to unveil the detailed composition of the extragalactic $\gamma$-ray sky above $\rm 100\,MeV$. In this paper, we derive the intrinsic source-count distribution (logN-logS) of extragalactic sources (i.e., blazars) at $|b|>20^{\circ}$ via the efficiency correction method. With this approach, we are able to measure the distribution down to a photon flux of $\sim10^{-10}$${\rm \, ph \,\, cm^{-2} \, s^{-1}}$ and to an energy flux of $\sim10^{-12}$${\rm \, erg \,\, cm^{-2} \, s^{-1}}$. In both cases the logN-logS becomes flatter at low fluxes. Moreover, we show that this logN-logS is representative of the blazar population (assuming the majority of unassociated sources are blazars) and allows us to constrain its evolution quite effectively. Among recently proposed evolutionary models, we find that the Pure Density Evolution (PDE) model best describes the evolutionary properties of the blazar population and that their integrated emission accounts for $\sim 50^{+10}_{-5}\%$ of the total extragalactic \gm-ray background.