2011ApJ...727..119W

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 x 10^–12 erg/s/cm2 (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 x10^–12 erg/s/cm2, 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.