The ROSAT-PSPC Spectrum of Bright Low-z Quasars

Details The ROSAT-PSPC Spectrum of Bright Low-z Quasars The ROSAT-PSPC Spectrum of Bright Low-z Quasars

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...457...77u&link_type=abstract

Astrophysical Journal v.457, p.77

Astronomy and Astrophysics

Astronomy

23

Galaxies: Quasars: General, X-Rays: Galaxies

Scientific paper

We present and discuss ROSAT Position Sensitive Proportional Counter (PSPC) spectra of seven bright low-redshift quasars obtained during pointed observations, including in the discussion the results previously obtained on the quasar PG 1116+215. Several pointed observations are available for two quasars. In two of our objects the count rate varied by a factor ˜2 during the pointed observations and we have extracted spectra at different flux levels. In all, we analyzed 10 PSPC spectra, plus the survey data.
The analysis leads to the following results: (1) In most cases, a single power law + absorption gives an adequate fit and the derived value of NH is consistent with the Galactic value. In these cases, the two- component models yield fits with similar reduced χ2 but do not bring statistically significant improvement to the fit. (2) Flux variations up to a factor of 2 occurred but were generally not accompanied by spectral shape variations. (3) The fastest variation observed was a decrease by a factor of 2 in 10 hr in PG 1404+226, which has a luminosity at high state of 1046 ergs s-1. This quasar also shows evidence for a strong absorption edge. (4) We show, through simulations, that if the total counts are less than approximately 4000-6000 genuine two-component spectra can be adequately fitted with a single power law. However, in the case of PG 1116+215, the very high statistics (12,000-25,000 counts) allow us to establish the lack of any significant contribution from a flat high-energy component. (5) Finally, we show that the ratio of UV to soft X-ray flux (measured at 0.4 keV) is not correlated with the spectral shape in the PSPC range. The correlation found by other authors between the ratio of UV to soft X-ray flux (measured at 2 keV) and the spectral shape is most probably caused by the hidden correlation between the normalization at 2 keV and the spectral slope in the PSPC range.

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