GINGA OBSERVATIONS OF THE X-RAY-SPECTRA OF SEYFERT-GALAXIES

We present an analysis of 60 spectra of 27 Seyfert galaxies observed w ith the Ginga Large Area proportional Counter (LAC). The 2-10 keV cont inuum is found to be compatible with previous spectral surveys, but a spectral flattening, or 'hard tail', is evident above 10 keV. Excess a bsorption over the Galactic column density is found in around half of the sources, with an equivalent hydrogen column density N(H) = 10(21-2 2) cm-2. Spectral features are found to be common, with all but two of the sources showing evidence for an iron Kalpha emission line. The me an energy of the line, at around 6.4 keV, indicates an origin via fluo rescence in near-neutral material. The mean equivalent width (100-150 eV) is large, considering that the sources are relatively unobscured. At least approximately 40 per cent of the sources show an additional i mprovement when an absorption edge is added, with an energy indicating an origin in highly ionized iron and equivalent hydrogen column densi ty N(HFe) of order 10(23) cm-2. These observations strongly imply the existence of two gas components -optically thick material and strongly photoionized gas of lower column density in the line of sight. The fi rst is responsible for the bulk of the emission line, and Compton down -scattering and absorption result in a 'reflected' continuum component which produces the hard tail. The mean line and reflected continuum p roperties are in excellent agreement with those expected in a flattene d geometry. Although other cold matter distributions are not ruled out , an origin in an accretion disc around the putative black hole seems most likely. The second component is highly ionized gas, the so-called 'warm absorber', which is responsible for the observed iron edge at 8 -9 keV and complex absorption at soft X-ray energies. This plasma is d ifficult to detect at other wavelengths, and may form an outflow or wi nd. Both the reflection component and the warm absorber considerably m odify the spectrum in the X-ray band, and previous observations with l ower signal-to-noise ratio have resulted in misleading estimates of th e spectrum of the underlying continuum. Rather than the GAMMA = 1.7 in dicated by HEAO-1 and EXOSAT, we find that the mean X-ray continuum is best modelled by a power law with photon index in the range GAMMA = 1 .9-2.0. These observations have considerable implications for the powe r-production mechanism in active galactic nuclei (AGN), and provide im portant diagnostics of the accretion flow and environment around the c entral black hole.