The soft γ-ray spectrum of galaxy clusters

Galaxy clusters appear to have a hard X-ray component based on recent SAX observations. The emission is best fit by a powerlaw and is likely non-thermal in origin. For the Coma cluster, the non-thermal emission is 10% of the thermal component in the 2-10 keV range and dominates above 25 keV. Thus it should be the dominant component in the soft gamma-ray regime. The spectral index is not well constrained by the current X-ray data but appears to be flatter than predicted by the standard Synchrotron-inverse Compton model for both Coma and A2256. INTEGRAL will be able to measure the spectral index of Coma, for example, to 20% and provide an important constraint on emission models. INTEGRAL observations should also reduce possible systematic errors associated with these early detections. The SAX observations challenge the standard model for non-thermal emission since some NT clusters have radio halos (Coma, A2256), others do not (A2199). Alternative emission processes have been suggested to explain the lack of...