The Lack of Diffuse, Nonthermal Hard X-ray Emission in the Coma Cluster: The Swift BAT's Eye View

TL;DR

This study uses Swift BAT data to search for diffuse nonthermal hard X-ray emission in the Coma cluster, finding no evidence and setting upper limits that imply a magnetic field strength greater than 0.2 microGauss.

Contribution

It introduces a method to extract fluxes of extended sources from Swift BAT data and applies it to constrain inverse Compton emission in the Coma cluster.

Findings

No evidence for large-scale IC emission at expected levels.

Upper limits on nonthermal flux are established, constraining magnetic field strength.

Results suggest magnetic fields in the Coma cluster are >0.2 microGauss.

Abstract

The Coma cluster of galaxies hosts the brightest radio halo known and has therefore been the target of numerous searches for associated inverse Compton (IC) emission, particularly at hard X-ray energies where the IC signal must eventually dominate over thermal emission. The most recent search with the Suzaku Hard X-ray Detector (HXD) failed to confirm previous IC detections with RXTE and BeppoSAX, instead setting an upper limit 2.5 times below their nonthermal flux. However, this discrepancy can be resolved if the IC emission is very extended, beyond the scale of the cluster radio halo. Using reconstructed sky images from the 58-month Swift BAT all sky survey, the feasibility of such a solution is investigated. Building on Renaud et al., we test and implement a method for extracting the fluxes of extended sources, assuming specified spatial distributions. BAT spectra are jointly fit with an XMM-Newton EPIC-pn spectrum derived from mosaic observations. We find no evidence for large-scale IC emission at the level expected from the previously detected nonthermal fluxes. For all nonthermal spatial distributions considered, which span the gamut of physically reasonable IC models, we determine upper limits for which the largest (most conservative) limit is <4.2x10^{-12} erg/s/cm^2 (20-80 keV), which corresponds to a lower limit on the magnetic field B>0.2uG. A nominal flux upper limit of <2.7x10^{-12} erg/s/cm^2, with corresponding B>0.25uG, is derived for the most probable IC distribution given the size of the radio halo and likely magnetic field radial profile.