X-rays and gamma-rays from radio-quiet Seyfert 1's and Cyg X-1
Andrzej A. Zdziarski
Copernicus Astronomical Center, Warsaw, Poland
Marek Gierlinski
Astronomical Observatory, Jagiellonian University, Cracow, Poland
Dorota Gondek
Copernicus Astronomical Center, Warsaw, Poland
Pawel Magdziarz
Astronomical Observatory, Jagiellonian University, Cracow, Poland
We review broad-band X-ray/\g-ray spectra of radio-quiet Seyfert 1 galaxies as
well as their theoretical implications. Average Seyfert 1's have intrinsic
power-law spectra with $\alpha \sim 0.9$ with a break at $\sim 0.3$--1 MeV,
and a superimposed Compton reflection component from cold matter covering
a $\sim 1$--$2\pi$ solid angle as seen by the power-law source. The reflection
component is responsible for a hardening of the spectrum in the
$\sim 10$--30 keV range and a softening at $\sim 30$--200 keV.
A rather similar spectrum is seen in the hard
state of Cyg X-1, a black-hole binary source. Simultaneous
\ginga/OSSE observations show $\alpha \simeq 0.6--0.7$, a strong Compton
reflection component, and a high-energy cutoff at $\sim 200$ keV.
The simplest model for the intrinsic spectra is thermal Comptonization of
soft seed photons. The scattering medium is relativistic ($kT\sim 200$ keV
or more), optically thin and probably forms a corona above a cold accretion
disk. However, some nonthermal models can also fit the data.
On the other hand, the X-ray spectra of some radio-quiet Seyferts
appear distinctly different from that above. NGC~4151 has a variable
$\alpha\sim 0.2$--0.7, no Compton reflection, and a high-energy break
corresponding to $kT\sim 50$ keV of an optically-thick Comptonizing plasma.
There is no Compton reflection seen in the spectra, which implies that the
hot source is not a disk corona.
The spectra of Seyfert 1's alone cannot account for the hard X-ray cosmic
background. However, the X-ray/\g-ray background can be explained by the sum of
contributions from evolved Seyfert 1's, Seyfert 2's, blazars, and, possibly,
supernovae type 1a.