# Searching for gamma-ray emission from galaxy clusters at low redshift

## TL;DR

This paper detects a significant cross-correlation between unresolved gamma-ray emission and galaxy clusters at low redshift, suggesting potential astrophysical sources like AGNs or intracluster medium emission.

## Contribution

It introduces a robust method for estimating the cross-correlation covariance matrix using mock realizations, applicable to future cross-correlation analyses.

## Key findings

- Detected a 3.5 sigma cross-correlation signal.
- Identified potential gamma-ray sources within galaxy clusters.
- Developed generalized mock data techniques for covariance estimation.

## Abstract

We report the identification of a positive cross-correlation signal between the unresolved $\gamma$-ray emission, measured by the \emph{Fermi} Large Area Telescope, and four different galaxy cluster catalogues. The selected catalogues peak at low-redshift and span different frequency bands, including infrared, optical and X-rays. The signal-to-noise ratio of the detected cross-correlation amounts to 3.5 in the most significant case. We investigate and comment about its possible origin, in terms of compact $\gamma$-ray emission from AGNs inside clusters or diffuse emission from the intracluster medium. The analysis has been performed by introducing an accurate estimation of the cross-correlation power-spectrum covariance matrix, built with mock realisations of the gamma and galaxy cluster maps. Different methods to produce the mock realizations starting from the data maps have been investigated and compared, identifying suitable techniques which can be generalized to other cross-correlation studies.