Search for cosmic-ray induced gamma-ray emission in Galaxy Clusters

TL;DR

This study searches for gamma-ray emission from galaxy clusters caused by cosmic-ray interactions, finding no significant detection but setting upper limits on cosmic-ray pressure and gamma-ray flux.

Contribution

First comprehensive search for gamma-ray emission in galaxy clusters using 4 years of Fermi-LAT data under a universal cosmic-ray model, establishing new constraints.

Findings

No significant gamma-ray detection; excess linked to background sources.

Limits on cosmic-ray to thermal pressure ratio below 1.4%.

Constraints on gamma-ray flux from individual clusters.

Abstract

Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into $\gamma$ rays, that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended $\gamma$-ray emission at the locations of 50 galaxy clusters in 4 years of Fermi-LAT data under the assumption of the universal cosmic-ray model proposed by Pinzke & Pfrommer (2010). We find an excess at a significance of $2.7\sigma$ which upon closer inspection is however correlated to individual excess emission towards three galaxy clusters: Abell 400, Abell 1367 and Abell 3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background (for example, radio galaxies within the clusters). Through the combined analysis of 50 clusters we exclude hadronic injection efficiencies in simple hadronic models above 21% and establish limits on the cosmic-ray to thermal pressure ratio within the virial radius, $R_{200}$, to be below 1.2-1.4% depending on the morphological classification. In addition we derive new limits on the $\gamma$-ray flux from individual clusters in our sample.