# Unresolved Gamma-Ray Sky through its Angular Power Spectrum

## TL;DR

This paper measures the angular power spectrum of unresolved gamma-ray background using eight years of Fermi LAT data, revealing evidence for two contributing source populations with distinct spectral properties.

## Contribution

It provides the first robust measurement of the UGRB anisotropy spectrum with enhanced sensitivity, supporting a two-source population model over a single-source scenario.

## Key findings

- Evidence (3.7σ) for two source populations contributing to UGRB
- A double power law with exponential cutoff fits the anisotropy spectrum well
- Photon indices for the two populations are 2.55 ± 0.23 and 1.86 ± 0.15

## Abstract

The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission (unresolved gamma-ray background, UGRB) below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This work presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with $\sim$3.7$\sigma$ significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55 $\pm$ 0.23 and 1.86 $\pm$ 0.15.