Galaxy Clusters in the Swift/BAT era: Hard X-rays in the ICM

TL;DR

This study detects 10 galaxy clusters in the hard X-ray band using Swift/BAT, finds no significant non-thermal emission in most, and provides new upper limits on inverse Compton emission and the first luminosity function above 15 keV.

Contribution

It presents the first logN-logS and luminosity function of galaxy clusters above 15 keV and constrains non-thermal emission mechanisms using multi-instrument data.

Findings

Most clusters do not show significant non-thermal hard X-ray emission.

Upper limits on inverse Compton emission challenge previous claims of excess.

Magnetic fields in some clusters may exceed 0.5 μG.

Abstract

We report about the detection of 10 clusters of galaxies in the ongoing Swift/BAT all-sky survey. This sample, which comprises mostly merging clusters, was serendipitously detected in the 15--55 keV band. We use the BAT sample to investigate the presence of excess hard X-rays above the thermal emission. The BAT clusters do not show significant (e.g. >2 $\sigma$) non-thermal hard X-ray emission. The only exception is represented by Perseus whose high-energy emission is likely due to NGC 1275. Using XMM-Newton, Swift/XRT, Chandra and BAT data, we are able to produce upper limits on the Inverse Compton (IC) emission mechanism which are in disagreement with most of the previously claimed hard X-ray excesses. The coupling of the X-ray upper limits on the IC mechanism to radio data shows that in some clusters the magnetic field might be larger than 0.5 $\mu$G. We also derive the first logN-logS and luminosity function distribution of galaxy clusters above 15 keV.