Broad band energy distribution of ROSAT detected quasars II: Radio-quiet objects

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 (〈Γgal〉 = 2.58±0.05 for z < 0.5) than radio-loud objects, with ∆Γ ∼ 0.4 and ∆Γ ∼ 0.3, compared to the flatand steep-spectrum radio quasars (Brinkmann et al. 1997), respectively. The spectral differences persist to high redshifts with ∆Γ ∼ 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 radioloud quasars. The spectral slopes of the ROSAT radioquiet quasars at z > 2.5 (Γ ∼ 2.23 −0.19) are consistent, within the errors, with those found for nearby quasars in the medium energy band (2–10 keV). 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., lx ∼ l e o, 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. Send offprint requests to: W. Yuan 1 Table I is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html The X-ray loudness αox appears to be independent of z, but regression analyses indicate a slight increase of αox 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 αox, objects which appear to be relatively “X-ray quiet” compared to the bulk of the other quasars.