Counting gamma rays in the directions of galaxy clusters

TL;DR

This study analyzed 52.5 months of FERMI-LAT gamma-ray data to detect signals from galaxy clusters, finding a marginal spatially extended gamma-ray signal likely dominated by AGNs, with limited evidence for pion decay contributions.

Contribution

The paper provides the first stacking analysis of gamma-ray emission from galaxy clusters above 10 GeV, setting upper limits on relativistic proton energy density and assessing the contributions of AGNs and pion decay.

Findings

Detected a 4.3 sigma gamma-ray signal in stacked cluster data.

Cool-core clusters show higher gamma-ray counts than non-cool core clusters.

Limited evidence for pion decay contribution, suggesting AGNs dominate the gamma-ray emission.

Abstract

Emission of AGNs and neutral pion decay - are the two most natural mechanisms, that could make a galaxy cluster be a source of gamma-rays in the GeV regime. We revisited this problem by using 52.5-month FERMI-LAT data above 10 GeV and stacking 55 clusters from the HIFLUGS sample of the X-ray brightest clusters. The choice of >10 GeV photons is optimal from the point of view of angular resolution, while the sample selection optimizes the chances of detecting signatures of the neutral pion decay, arising from hadronic interactions of relativistic protons with an intra-cluster medium, which scale with the X-ray flux. In the stacked data we detected a signal for the central 0.25 deg circle at the level of 4.3 sigma. An evidence for a spatial extent of the signal is marginal. A subsample of cool-core clusters has higher count rate 1.9+/-0.3 per cluster compared to the subsample of non-cool core clusters 1.3+/-0.2. Several independent arguments suggest that the contribution of AGNs to the observed signal is substantial if not dominant. No strong support for the large contribution of pion decay was found. In terms of a limit on the relativistic protons energy density, we got an upper limit of ~1.5% relative to the gas thermal energy density, provided that the spectrum of relativistic protons is hard (s=4.1 in dN/dp=p^-s). This estimate assumes that relativistic and thermal components are mixed. For softer spectra the limits are weaker.