The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars. II. Final Results
Abstract
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 NGalHI<1.9×1020 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 × 1019 cm-2 on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are ~1 × 1019 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 Å 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 <~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. 4. Extensive correlation analysis of the X-ray continuum emission parameters with optical emission-line parameters indicates that the strongest correlation is between the spectral slope αx and the Hβ FWHM. A possible explanation for this remarkably strong correlation is a dependence of αx on L/LEdd, as seen in Galactic black hole candidates. 5. The strong correlations between αx and L[O III], Fe II/Hβ, and the peak [O III] to Hβ flux ratio are verified. The physical origin of these correlations is still not understood. 6. There appears to be a distinct class of ``X-ray-weak'' quasars, which form ~10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10-30, than expected based on their luminosity at other bands and on their Hβ luminosity. These may be quasars in which the direct X-ray source is obscured and only scattered X-rays are observed. 7. Thin accretion disk models cannot reproduce the observed 0.2-2 keV spectral shape, and they also cannot reproduce the tight correlation between the optical and soft X-ray emission. An as yet unknown physical mechanism must be maintaining a strong correlation between the optical and soft X-ray emission. 8. The H I/He I ratio in the high Galactic latitude ISM must be within 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 interstellar medium is mostly ionized to He II or He III. We finally note the intriguing possibility that although <αx> 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 of radio emission. The difference in <αx> may result from the strong αx versus Hβ FWHM correlation and the tendency of radio-loud quasars to have broader Hβ.- Publication:
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The Astrophysical Journal
- Pub Date:
- March 1997
- DOI:
- 10.1086/303696
- arXiv:
- arXiv:astro-ph/9609164
- Bibcode:
- 1997ApJ...477...93L
- Keywords:
-
- Galaxies: Active;
- Galaxies: Nuclei;
- Galaxies: Quasars: General;
- X-Rays: Galaxies;
- Astrophysics
- E-Print:
- 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol. 447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8 figures available at http://physics.technion.ac.il/~laor/rosat/paper.ps