THE X-RAY-SPECTRA OF HIGH-LUMINOSITY ACTIVE GALACTIC NUCLEI OBSERVED BY GINGA

Results are presented on the X-ray emission from t3 objects, observed by Ginga as part of a spectral survey of bright quasars in the energy range 2-20 keV. The distribution of the power-law energy index for this sample has a mean of alpha-BAR = 0.81 and shows significant intrinsic dispersion, sigma = 0.31, values which are broadly compatible with those from samples of lower luminosity active galactic nuclei. The mean spectral slope is clearly too steep to fit the spectrum of the cosmic diffuse X-ray background (CXB) at energies less than 20 keV, confirming the "spectral paradox" in the discrete-source explanation of the CXB. We have searched for correlations between the X-ray spectral properties and those in other frequency bands, and have found a possible connection between the near-infrared spectral indices and the X-ray properties of quasars. There is also evidence that quasars with steep X-ray spectra are radio-quiet, in support of the previous results obtained for the 0.3-3.5 keV band; our results, however, are compatible with a single power-law component in the 0.3-10 keV range, in contrast to some previous results. The first evidence for departures from a simple power-law continuum in Mrk 205 is revealed; this, together with previously reported evidence for iron lines in other quasars, suggests that an appreciable fraction of quasars may possess iron lines with equivalent widths approximately 100 eV. Models involving reprocessing of continuum flux in cool material near the core, successful in the case of Seyfert galaxies, fit well for the lower luminosity objects in our sample; for the high-luminosity objects the fit is not as good, and other processes, such as beaming, may be implicated.