1998A&A...330..108Y

A database of radio-quiet quasars detected with ROSAT is presented containing 846 quasars seen in the All-Sky Survey and/or in pointed PSPC observations. About ∼70% of the objects have been detected in X-rays for the first time. We present the soft X-ray fluxes and spectra, if available. Using an optically selected subsample compiled from this database we study the broad band properties of radio-quiet quasars with high statistical significance. We confirm that radio-quiet quasars have in general steeper soft X-ray spectra (<{GAMMA}_gal≥2.58±0.05 for z<0.5) than radio-loud objects, with {DELTA}{GAMMA}∼0.4 and {DELTA}{GAMMA}∼0.3, compared to the flat- and steep-spectrum radio quasars (Brinkmann et al. 1997), respectively. The spectral differences persist to high redshifts with {DELTA}{GAMMA}∼0.6 at z>2 compared to flat-spectrum radio-loud quasars. A spectral flattening with redshift is confirmed for the radio-quiet objects up to z∼2, beyond which the spectral slopes seem to be independent of redshift, similar to that found for radio-loud quasars. The spectral slopes of the ROSAT radio-quiet quasars at z>2.5 ({GAMMA}∼2.23^+0.16_–0.19) are consistent, within the errors, with those found for nearby quasars in the medium energy band (2-10keV). This implies that X-ray spectral evolution is not important in radio-quiet quasars. We show that there is, in a statistical sense, little or no excess absorption for most of the radio-quiet objects at z > 2, in contrast to their radio-loud counterparts. By dividing the sample into narrow redshift bins, the existence of a correlation between the X-ray luminosity and the luminosity at 2500Å, i.e., l_x∼l_o^e, is confirmed. Individual objects show a large scatter from this correlation and the slope e takes values in the range 0.75≲e≲1.17, depending on the mathematical method used to analyze the data. The X-ray loudness α_ox appears to be independent of z, but regression analyses indicate a slight increase of α_ox with optical luminosity. However, this behavior is, very likely, not caused by physical properties inherent to the quasars but is the result of the intrinsic dispersion of the luminosities and the flux limits in both the optical and X-ray observations. Finally, we find a small fraction of sources with a substantially larger value of α_ox, objects which appear to be relatively ``X-ray quiet'' compared to the bulk of the other quasars.