The hard X-ray emission of quasars

The analysis of observations of 50 radio-loud and radio-quiet quasars made by the X-ray satellite Ginga is presented. The spectra, the most accurate and sensitive yet in the 2-20 keV band, provide a unique opportunity to study the hard X-ray emission from these faint, though highly luminous objects. The properties of the X-ray continuum are compatible with previous studies, and reveal a significant measured spread on the spectral index, which is unlikely to be due to spectral variability or to the radio heterogenity of the sample, indicating a true variety of spectral index amongst objects. The flux variability shown by the radio-loud objects is greater than that shown by the radio-quiet objects, but there is no indication of any differences in spectral variability, which is generally low. There is a strong relationship present between the X-ray spectral index and radio loudness, with radio-quiet quasars on average having softer spectra. The spectral index of the radio- loud objects appears to depend on orientation, with objects having the jet closer to sky plane showing steeper spectra than those pointed more towards the Earth. A number of the spectra show features, with non-Galactic absorbtion and Fe Ka line emission being detected. Spectral flattening above 10 keV reported for Seyfert galaxies and attributed to Compton 'reflection' is not strongly supported. The observed difference in the spectral distributions of the radio-quiet quasars and Seyfert galaxies can be accounted for by the absence of warm absorbing material in some objects. The results presented are consistent with current unified models, and with the idea of relativistically beamed X-ray emission in the radio-loud objects. It is likely that more than one mechanism contributes to the beamed emission, with the spectra of these objects depending on orientation, and on the relative luminosities of the non-thermal and thermal continua.