Physical processes in the X-ray/gamma-ray source of IC 4329A

Theoretical implications of the recent simultaneous ROSAT/OSSE observation of the Seyfert 1 IC 4329A are discussed. We find that either thermal or non-thermal Comptonization models can fit the data. Thermal models require optically thin plasma and mildly relativistic electrons. Non-thermal models require most of the injected power to be at low values of the electron Lorentz factor. Both models predict a spectral cut-off at energies below ~1 MeV, which agrees with broad-band data including non- simultaneous Ginga spectra that match the ROSAT 2-keV flux. Also, Compton reflection corresponding to cold matter covering a solid angle of 2π is required, and partial covering is ruled out. Since the spectrum of IC 4329A is typical of Seyfert 1s, their contribution 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 the vF_v_ spectrum of IC 4329A is much broader than that of the hard X-ray background, however, Seyfert 1s cannot account for the X-ray background spectrum without spectral evolution.