EXOSAT X-RAY-SPECTRA OF QUASARS

We have measured the X-ray spectra in the 0.1-10 keV energy range for 17 AGNs (mostly PG quasars) using the EXOSAT Low and Medium Energy arrays. The sample has been selected on the basis of soft X-ray flux and includes all the AGNs in the 1981 Zamorani et al. and 1986 Tananbaum et al. publications with an Einstein flux greater than 6 x 10(-12) ergs cm-2 s-1. For each object we have determined the best-fit spectral parameters in the medium-energy range (approximately 2-10 keV), i.e., the power-law slope, the normalization, and the absorbing column density. In this energy range, all the spectra are well described by a single power law model, with a wide distribution of energy spectral indices in the range 0.4 < alpha < 1.3. The average spectral index is <alpha> = 0.89 +/- 0.06, but most of the objects are distributed around alpha congruent-to 1.0. The overall distribution of spectral indices is significantly displaced toward steeper spectra, when compared to that found in the same energy band by Turner & Pounds, for a large sample of hard X-ray-selected Seyfert galaxies. This difference can be understood in terms of the different selection criteria of the two samples (soft versus hard X-ray selection), and it is concluded that the true distribution of hard X-ray spectral indices in AGNs is probably wider than previously suggested. Inclusion of lower energy data (approximately 0.1-2.0 keV) shows the presence of significant soft excess in six out of 17 cases. This soft excess emission can be modeled by either a steep power law with alpha a ranging from 2.7 to 4.2 or the high-energy tail of a blackbody with temperatures in the range 40-80 eV. The intersection of this soft excess emission with the hard X-ray power law is in the energy interval 0.35-0.75 keV, in the source rest frame. A correlation analysis performed on our data set does not reveal any correlation between the medium-energy spectral properties and other physical parameters of the sources. However, fits performed in the energy range 0.1-4.0 keV (comparable with the Einstein IPC energy range) show a significant correlation between radio-loudness and X-ray spectral properties, with the radio-loud objects having, on average, flatter slopes than the radio-quiet ones, in agreement with the findings of Wilkes & Elvis. The fact that the same correlation is not seen in our data at higher energy seems to be explained by a correlation between presence and strength of soft excess and "radio-quietness": the objects with significant soft excess emission tend to be the most radio-quiet quasars in our sample.