THE LACK OF DIFFUSE, NON-THERMAL HARD X-RAY EMISSION IN THE COMA CLUSTER: THESWIFTBURST ALERT TELESCOPE'S EYE VIEW

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 failed to confirm previous IC detections with RXTE and BeppoSAX, instead setting an upper limit 2.5 times below their non-thermal 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 Burst Alert Telescope (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 non-thermal fluxes. For all non-thermal spatial distributions considered, which span the gamut of physically reasonable IC models, we determine upper limits for which the largest (most conservative) limit is ≲4.2 × 10−12 erg s−1 cm−2 (20–80 keV), which corresponds to a lower limit on the magnetic field B > 0.2 μ G. A nominal flux upper limit of <2.7 × 10−12 erg s−1 cm−2, with corresponding B > 0.25 μ G, is derived for the most probable IC distribution given the size of the radio halo and likely magnetic field radial profile.