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| Title: |
The soft x-ray properties of a complete sample of optically selected quasars. 1: First results
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| Author(s): |
Laor, Ari; Fiore, Fabrizio; Elvis, Martin; Wilkes, Belinda J.; Mcdowell, Jonathan C.
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| 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.
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| NASA Center: |
Goddard Space Flight Center; Marshall Space Flight Center
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| Publication Date: |
November 10, 1994
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| Document Source: |
Other Sources |
| No Digital Version Available: |
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| Document ID: |
19950038819
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| Accession ID: |
95A70418
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| Publication Information: |
Astrophysical Journal, Part 1, 435, 2, p. 611-630, ISSN 0004-637X, Number of Pages = 20 |
| Contract-Grant-Task Number: |
NAG5-1618 |
| Keywords: |
GALACTIC RADIATION; QUASARS; X RAY ASTRONOMY; X RAY SPECTRA; FLUX DENSITY; RED SHIFT; ROSAT MISSION; STELLAR LUMINOSITY;
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| Notes: |
Astrophysical Journal, Part 1 vol. 435, no. 2 p. 611-630 November 10, 1994 |
| Accessibility: |
Unclassified; Copyright; Unlimited; Publicly available; |
| Updated/Added to NTRS: |
2004-11-03 |
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