Ginga observations of the X-ray spectra of Seyfert galaxies

Abstract

We present an analysis of 60 spectra of 27 Seyfert galaxies observed with the Ginga Large Area proportional Counter (LAC). The 2–10 keV continuum is found to be compatible with previous spectral surveys, but a spectral flattening, or ‘hard tail’, is evident above 10 keV. Excess absorption over the Galactic column density is found in around half of the sources, with an equivalent hydrogen column density |$N_{\rm H}=10^{21-22} \ {\rm {cm}}^{-2}$|⁠, Spectral features are found to be common, with all but two of the sources showing evidence for an iron |${\rm K}\alpha$| emission line. The mean energy of the line, at around 6.4 keV, indicates an origin via fluorescence in near-neutral material. The mean equivalent width (100–150 eV) is large, considering that the sources are relatively unobscured. At least |$\sim 40$| per cent of the sources show an additional improvement when an absorption edge is added, with an energy indicating an origin in highly ionized iron and equivalent hydrogen column density |$N_{\rm {HFe}}$| of order |$10^{23} \ {\rm {cm}}^{-2}$|⁠, 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 first 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 properties are in excellent agreement with those expected in a flattened 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 difficult to detect at other wavelengths, and may form an outflow or wind. Both the reflection component and the warm absorber considerably modify the spectrum in the X-ray band, and previous observations with lower signal-to-noise ratio have resulted in misleading estimates of the spectrum of the underlying continuum. Rather than the |$\Gamma=1.7$| indicated 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 power-production mechanism in active galactic nuclei (AGN), and provide important diagnostics of the accretion flow and environment around the central black hole.