X-ray to gamma-ray virial shock signal from the Coma cluster

TL;DR

This study detects X-ray and gamma-ray signals from the virial shock of the Coma cluster, providing evidence of cosmic-ray acceleration and energy deposition at the cluster's outskirts.

Contribution

First multi-wavelength detection of virial shock signatures in X-ray and gamma-ray bands for the Coma cluster, confirming theoretical predictions.

Findings

3.4σ LAT excess aligned with VERITAS ring

>5σ ROSAT detection matching expected shock signature

Consistent energy deposition of ~0.3% in cosmic-ray electrons

Abstract

Following evidence for an east--west elongated virial ring around the Coma galaxy cluster in a $\sim220$ GeV VERITAS mosaic, we search for corresponding signatures in $>$GeV $\gamma$-rays from Fermi-LAT, and in soft, $\sim0.1$ keV X-rays from ROSAT. For the ring elongation and orientation inferred from VERITAS, we find a $3.4\sigma$ LAT excess, and detect ($>5\sigma$) the expected signature in ROSAT bands R1 and R1+R2. The significances of both LAT and ROSAT signals are maximal near the VERITAS ring parameters. The intensities of the ROSAT, Fermi, and VERITAS signals are consistent with the virial shock depositing $\sim0.3\%$ (with an uncertainty factor of $\sim3$) of its energy over a Hubble time in a nearly flat, $p\equiv - d\ln N_e/d\ln E\simeq 2.0$--$2.2$ spectrum of cosmic-ray electrons. The sharp radial profiles of the LAT and ROSAT signals suggest preferential accretion in the plane of the sky, as indicated by the distribution of neighboring large-scale structure. The X-ray signal gauges the compression of cosmic-rays as they are advected deeper into the cluster.