Abstract
We present the results of ROSAT position sensitive proportional counter (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 less than or = 0.400 and a Galactic H I column density N(sup Gal sub H I) less than 1.9 x 10(exp 20)/sq cm. These selection criteria, combined with the high sensitivity and improved energy resolution of the PSPC, allow us to determine the soft (approximately 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 alpha(sub x) and the following emission line parameters: H beta Full Width at Half Maximum (FWHM), L(sub O III), and the Fe II/H beta flux ratio. These correlations imply the following: (1) The quasar's environment is likely to be optically thin down to approximately 0.2 keV. (2) In most objects alpha(sub x) varies by less than approximately 10% on timescales shorter than a few years. (3) alpha(sub 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 approximately 30%. We find mean value of alpha(sub 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 micrometer(s) flux is given. This implies that the X-ray variability power spectra of quasars flattens out between f approximately 10(exp -5) and f approximately 10(exp -8) Hz. A steep alpha(sub 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.
Highlights
Quasars emit most of their power in the ultraviolet to soft X-ray regime
The position-sensitive proportional counter (PSPC) detector aboard ROSAT allowed a signiÐcantly improved study of the soft X-ray emission of quasars compared with earlier missions, such as HEAO-1, Einstein, EXOSAT, and Ginga
To overcome the potential biases in existing studies, we initiated a ROSAT PSPC program to make an accurate determination of the soft X-ray properties of a well-deÐned and complete sample of quasars, selected independently of their X-ray properties
Summary
Quasars emit most of their power in the ultraviolet to soft X-ray regime. The position-sensitive proportional counter (PSPC) detector aboard ROSAT allowed a signiÐcantly improved study of the soft X-ray emission of quasars compared with earlier missions (some of which were not sensitive below 2 keV), such as HEAO-1, Einstein, EXOSAT , and Ginga To overcome the potential biases in existing studies, we initiated a ROSAT PSPC program to make an accurate determination of the soft X-ray properties of a well-deÐned and complete sample of quasars, selected independently of their X-ray properties. This program was designed to address the following questions : 1. The additional advantages of the BQS sample are that it has been extensively explored at other wavelengths (see Laor et al 1994, hereafter Paper I, for further details) and that it includes only bright quasars, allowing high signal-to-noise (S/N) X-ray spectra for most objects. We conclude in ° 5 with answers to the questions raised above, and with some new questions to be addressed in future studies
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