Swift BAT Upper Limits on Non-thermal Hard X-ray Emission from Clusters of Galaxies

Swift BAT is a coded aperture instrument covering 1/8 of the whole sky in each pointing. While it is waiting for gamma-ray bursts at various pointing directions, the BAT accumulates all events from the sky, essentially performing an all sky survey. In the 22-month BAT all sky survey, we have detected 10 clusters of galaxies at >5sigma Perseus, Coma, Ophiuchus, Cygnus A, A2319, A754, A3266, A2142, A3571, Triangulum Australis. The BAT spectra of Perseus, Cygnus A, and A2142 in the 14-100 keV band are dominated by the AGN component in or around the clusters. The other 7 clusters are all hot (kT 10 keV) and their BAT spectrum is consistent with an extension of the thermal emission modeled with ASCA archival data in the 2-10 keV band, and does not require any additional component. In other words, there is no evidence of a non-thermal diffuse component in these clusters. A combined spectrum of the merging clusters shows a small indication of the existence of non-thermal emission compared with the relaxed clusters. However it can be also explained by a thermal model (kT 6.7 keV) with slightly higher temperature than weighted average (=5.8

keV). We estimated the upper limit of the non-thermal emission by adding a power-law model to the spectral fit for the 10 detected clusters. The upper limit is 6x10^-12 ergs/cm²/s in average and the lower limit of the magnetic field B ranges 0.2-1 uG assuming inverse Compton scattering of Cosmic Microwave Background photons by relativistic electrons in the cluster. This magnetic field is slightly higher than the value reported by BeppoSAX, but is consistent with variation of B over the cluster obtained by Faraday rotation, since the BAT spectrum is dominated by flux from the 30 arcmin around the cluster center, while BeppoSAX collected most of the cluster flux.