PHYSICAL PROCESSES IN THE X-RAY GAMMA-RAY SOURCE OF IC-4329A

Theoretical implications of the recent simultaneous ROSAT/OSSE observa tion of the Seyfert 1 IC 4329A are discussed. We find that either ther mal or non-thermal Comptonization models can fit the data. Thermal mod els require optically thin plasma and mildly relativistic electrons. N on-thermal models require most of the injected power to be at low valu es of the electron Lorentz factor. Both models predict a spectral cut- off at energies below approximately 1 MeV, which agrees with broad-ban d data including non-simultaneous Ginga spectra that match the ROSAT 2 -keV flux. Also, Compton reflection corresponding to cold matter cover ing a solid angle of 2pi is required, and partial covering is ruled ou t.Since the spectrum of IC 4329A is typical of Seyfert 1 s, their cont ribution to the cosmic hard X-ray background can be estimated: without cosmological evolution, it is 30 per cent at 3 keV. As the peak of th e nuF(nu) spectrum of IC 4329A is much broader than that of the hard X -ray background, however, Seyfert Is cannot account for the X-ray back ground spectrum without spectral evolution.