The soft X-ray excess in Einstein quasar spectra.
Abstract
We study the low-energy excess above a high-energy power law in the X-ray spectra for a signal-to-noise limited, subsample of 14 quasars from the Wilkes & Elvis (1987) sample. Detailed analysis of the Einstein imaging proportional counter (IPC) data, combined with monitor proportional counter (MPC) data where possible, and the use of accurate Galactic N_H_ values allow us to estimate the flux of any detected excess and improve constraints on the spectral parameters at higher (>0.6 keV) X-ray energies. We find a significant excess in 8 of the 14 objects, confined in all but one case to below 0.6 keV. The excess is typically strong, dominating the <~0.3 keV X-ray flux and being 1-6 times as strong as the high-energy component at 0.2 keV. In 3C 273, multiple observations show that the excess is variable. The lack of spectral information, uncertainty in the intrinsic line-of-sight absorption and the possibility of variation prevents the detection of any relation with the optical/ultraviolet blue bump. Comparison with an existing accretion disk model for the blue bump suggests that, for this model to explain both the blue bump and the soft X-ray excess, a range of disk parameters are required to explain the whole sample.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- January 1992
- Bibcode:
- 1992A&A...253...35M
- Keywords:
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- Astronomical Spectroscopy;
- Quasars;
- Stellar Spectra;
- X Ray Astronomy;
- X Ray Spectroscopy;
- Proportional Counters;
- Spectrum Analysis;
- X Ray Spectra;
- Astrophysics