Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...435..611l&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 435, no. 2, p. 611-630
Astronomy and Astrophysics
Astronomy
122
Galactic Radiation, Quasars, X Ray Astronomy, X Ray Spectra, Flux Density, Red Shift, Rosat Mission, Stellar Luminosity
Scientific paper
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 NGalH I less than 1.9 x 1020/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 alphax and the following emission line parameters: H beta Full Width at Half Maximum (FWHM), LO 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 alphax varies by less than approximately 10% on timescales shorter than a few years. (3) alphax 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 alphax = -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 10-5 and f approximately 10-8 Hz. A steep alphax 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.
Elvis Martin
Fiore Fabrizio
Laor Ari
McDowell Jonathan C.
Wilkes Belinda J.
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