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V. Petrosian (Stanford University)
BeppoSAX detection of hard X-rays (20-100 keV) from Coma and A2256 clusters (Fusco-Femiano et al., ApJL 1999, 513, L21 and 2000, 534, L7) has led to a flurry of theoretical interpretations. The first of two possible mechanisms for production of this radiation is the inverse Compton scattering of cosmic microwave photons by relativistic electrons (Lorentz factor \gamma > 103), which also produce the observed diffuse radio emission via the synchrotron mechanism. This (and a similar interpretation for the observed EUV emission; Bowyer & Berghofer, ApJ 1998, 516, 502) require a low magnetic field (B < 0.2 \mu{\rm G}) which is in conflict with the expected field of about 2 \mu{\rm G} based on Faraday rotation measured in other clusters. This discrepancy has led several authors to propose a second possible mechanism, namely bremsstrahlung by semi relativistic electrons (energy > 20 keV). It will be shown that this model also is seriously flawed because it requires a huge input of energy into the inter cluster gas which could rapidly heat the background plasma to higher than observed temperatures. Alternative models will be presented and their merits and disadvantages, and the mechanisms of the acceleration of electrons will be described.