Evolution of Hard X-Ray Radiation from Clusters of Galaxies:   Bremsstrahlung or Inverse Compton Scattering?
  We have calculated evolution of a non-thermal electron population from super-thermal but weakly relativistic to highly relativistic energy range in clusters of galaxies. We investigate evolution of hard X-ray radiation due to both bremsstrahlung and inverse Compton scattering of the cosmic microwave background photons. The bremsstrahlung component is more significant than the inverse Compton scattering one when the momentum spectra of electron sources are steeper than $\sim P_e^{-3.0}$ and vice versa in the case of Coma, where $P_e$ is an electron momentum. The resultant hard X-ray spectra are flatter when the bremsstrahlung component is dominant. When the spectral indices of the source term are in the intermediate range ($ -2.5 \sim -3.5$), too much extreme ultraviolet emission is produced. Inverse Compton dominant models can reproduce Coma cluster results with reasonable injection rates, which are possible in cluster mergers and/or ambient gas accretion. 
Motokazu Takizawa (Yamagata Univ., Univ. of Virginia)