Broad band energy distribution of ROSAT detected quasars

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 radioloud 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 radio-loud quasars. The spectral slopes of the ROSAT radioquiet quasars at z > 2.5 (Γ ∼ 2.23+0.16 −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 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. The X-ray loudness αox appears to be independent of z, but regression analyses indicate a slight increase ofαox with optical luminosity. However, this behavior is, very likely, not caused by physical properties inherent to the quasars but is the result of Send offprint requests to: W. Yuan ? Table 1 is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.ustrasbg.fr/Abstract.html 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.