Particle acceleration and origin of the hard X-rays in halos of clusters of galaxies

We analyse models of hard X-ray production in clusters of galaxies. The inverse Compton origin of these Xrays seems to be problematic since it requires rather specific parameters for the photon production. The model of bremsstrahlung radiation by suprathermal electrons if they are accelerated from the thermal pool requries a characteristic acceleration time of the order of 10(15) sec to explain the observed X-ray flux from the Coma cluster. In the case of proton bremsstrahlung origin a more conservative acceleration compared to that of electrons is sufficient to explain the X-ray data. In the last case the Xray emission should be accompanied by nuclear gamma-ray line flux.