The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars. I. First Results
Abstract
We present the results of ROSAT PSPC observations of 10 quasars. These objects are part of our ROSAT program to observe a complete sample of optically selected quasars. This sample includes all 23 quasars from the bright quasar survey with a redshift z <= 0.400 and a Galactic H I column density N_HI_^Gal^ < 1.9 x 10^20^ cm^-2^. These selection criteria, combined with the high sensitivity and improved energy resolution of the PSPC, allow us to determine the soft (~0.2-2 keV) X-ray spectra of quasars with about an order of magnitude higher precision compared with earlier soft X-ray observations. The following main results are obtained: Strong correlations are suggested between the soft X-ray spectral slope α_x_ and the following emission line parameters: HβFWHM, L_[O III}_, and the Fe II/Hβ flux ratio. These correlations imply the following: (1) The quasar's environment is likely to be optically thin down to ~0.2 keV. (2) In most objects α_x_ varies by less than ~10% on timescales shorter than a few years. (3)α_x_ might be a useful absolute luminosity indicator in quasars. (4) The Galactic He I and H I column densities are well correlated. Most spectra are well characterized by a simple power law, with no evidence for either significant absorption excess or emission excess at low energies, to within ~30%. We find <α_x_> = - 1.50+/-0.40, which is consistent with other ROSAT observations of quasars. However, this average is significantly steeper than suggested by earlier soft X-ray observations of the Einstein IPC. The 0.3 keV flux in our sample can be predicted to better than a factor of 2 once the 1.69 micron flux is given. This implies that the X-ray variability power spectra of quasars flattens out between f~10^-5^ and f ~ 10^-8^ Hz. A steep α_x_ is mostly associated with a weak hard X-ray component, relative to the near-IR and optical emission, rather than a strong soft excess, and the scatter in the normalized 0.3 keV flux is significantly smaller than the scatter in the normalized 2 keV flux. This argues against either thin or thick accretion disks as the origin of the soft X-ray emission. Further possible implications of the results found here are briefly discussed.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1994
- DOI:
- 10.1086/174841
- Bibcode:
- 1994ApJ...435..611L
- Keywords:
-
- Galactic Radiation;
- Quasars;
- X Ray Astronomy;
- X Ray Spectra;
- Flux Density;
- Red Shift;
- Rosat Mission;
- Stellar Luminosity;
- Astronomy;
- ACCRETION;
- ACCRETION DISKS;
- ISM: ABUNDANCES;
- GALAXIES: QUASARS: GENERAL;
- X-RAYS: GALAXIES