The soft x-ray properties of a complete sample of optically selected quasars. 1: 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.