THE SOFT X-RAY PROPERTIES OF A COMPLETE SAMPLE OF OPTICALLY SELECTED QUASARS. Ⅱ. FINAL RESULTS

摘要

We present the final results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low-z quasars. This sample includes all 23 quasars from the Bright Quasar Survey with z ≤ 0.400 and N_(H I)~(Gal) < 1.9 x 10~(20) cm~(-2). Pointed ROSAT PSPC observations were made for all quasars, yielding high signal-to-noise (S/N) spectra for most objects, which allowed an accurate determination of the spectral shape. The following main results were obtained: 1. The spectra of 22 of the 23 quasars are consistent, to within ～30%, with a single power-law model at rest-frame 0.2-2 keV. There is no evidence for significant soft excess emission with respect to the best-fit power law. We place a limit (95% confidence) of ～5 x 10~(19) cm~(-2) on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are ～1 x 10~(19) cm~(-2) in the two highest S/N spectra. 2. The mean 0.2-2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050-350 A continuum recently determined by Zheng et al. (1996) for z > 0.33 quasars. This suggests that there is no steep soft component below 0.2 keV. 3. Significant X-ray absorption (τ > 0.3) by partially ionized gas (" warm absorber") in quasars is rather rare, occurring for approx < 5% of the population, which is in sharp contrast to lower luminosity active galactic nuclei (AGNs), where significant absorption probably occurs for ～ 50% of the population.