Observations and theory of X-rays and gamma-rays from radio-quiet seyferts

Abstract

Recent OSSE observations of soft γ-ray emission from radio-quiet Seyfert galaxies as well as their theoretical implications are reviewed. For some Seyferts, the OSSE data can be combined with X-ray data from Ginga and ROSAT. Seyfert 1s have intrinsic power-law spectra with α ∼ 0.9 extending without a break to at least ∼ 200 keV. There is some evidence for a break at higher energies. On top of this intrinsic spectrum there is a Compton reflection component, which corresponds to cold matter covering a ∼ 2π solid angle as seen by the power-law source. Compton reflection is responsible for a hardening of the spectrum in the ∼ 5–30 keV range and a softening at ∼ 30–200 keV. This spectrum is then absorbed by a partially ionized external medium. The spectra of Seyfert 2s are marginally consistent with those of Seyfert 1s modified by strong absorption. However, there are indications that their intrinsic X-ray spectra are harder than those of Seyfert 1s. This is in fact the case for NGC 4151, a Seyfert 1.5. The Seyfert spectra integrated over redshift with cosmological evolution can explain the cosmic X-ray background spectrum from 1 to 100 keV. The simplest model for the intrinsic Seyfert spectra is thermal Comptonization of UV photons. The scattering medium in Seyfert 1s is optically thin and relativistic whereas in NGC 4151 and possibly in Seyfert 2s it is optically thick and nonrelativistic. Nonthermal models with the power injected in electrons with low Lorentz factors are also possible.