THE SOFT-X-RAY PROPERTIES OF A COMPLETE SAMPLE OF OPTICALLY SELECTED QUASARS .2. FINAL RESULTS

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 l ess than or equal to 0.400 and N-HI(Gal) < 1.9 x 10(20) cm(-2). Pointe d ROSAT PSPC observations were made for all quasars, yielding high sig nal-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 wi thin similar to 30%, with a single power-law model at rest-frame 0.2-2 keV. There is no evidence for significant soft excess emission with r espect to the best-fit power law. We place a limit (95% confidence) of similar to 5 x 10(19) cm(-2) on the amount of excess foreground absor ption by cold gas for most of our quasars. The limits are similar to 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 Angstrom continuum recently determined by Zheng et a l. (1996) for z > 0.33 quasars. This suggests that there is no steep s oft component below 0.2 keV. 3. Significant X-ray absorption (tau > 0. 3) by partially ionized gas (''warm absorber'') in quasars is rather r are, occurring for less than or similar to 5% of the population, which is in sharp contrast to lower luminosity active galactic nuclei (AGNs ), where significant absorption probably occurs for similar to 50% of the population. 4. Extensive correlation analysis of the X-ray continu um emission parameters with optical emission-line parameters indicates that the strongest correlation is between the spectral slope alpha(x) and the H beta FWHM. A possible explanation for this remarkably stron g correlation is a dependence of alpha(x) on L/L(Edd), as seen in Gala ctic black hole candidates. 5. The strong correlations between alpha(x ) and L([O III]), Fe II/H beta, and the peak [O III] to H beta flux ra tio are verified. The physical origin of these correlations is still n ot understood. 6. There appears to be a distinct class of ''X-ray-weak '' quasars, which form similar to 10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10-30, than e xpected based on their luminosity at other bands and on their H beta l uminosity. These may be quasars in which the direct X-ray source is ob scured and only scattered X-rays are observed. 7. Thin accretion disk models cannot reproduce the observed 0.2-2 keV spectral shape, and the y also cannot reproduce the tight correlation between the optical and soft X-ray emission. An as yet unknown physical mechanism must be main taining a strong correlation between the optical and soft X-ray emissi on. 8. The H I/He I ratio in the high Galactic latitude ISM must be wi thin 20%, and possibly within 5%, of the total H/He ratio of 10, which indicates that He in the diffuse H II gas component of the interstell ar medium is mostly ionized to He II or He III. We finally note the in triguing possibility that although [alpha(x)] in radio-loud quasars (- 1.15 +/- 0.14) is significantly flatter than in radio-quiet quasars (- 1.72 +/- 0.09) the X-ray emission may not be related to the presence o f radio emission. The difference in [alpha(x)] may result from the str ong alpha(x) versus H beta FWHM correlation and the tendency of radio- loud quasars to have broader H beta.