X‐Ray Spectral Survey of WGACAT Quasars. I. Spectral Evolution and Low‐Energy Cutoffs

Abstract

We have used the WGA catalog of ROSAT Position Sensitive Proportional Counter (PSPC) X-ray sources to study the X-ray spectrum of about 500 quasars in the redshift interval 0.1-4.1, detected with a signal-to-noise ratio better than 7. We have parameterized the PSPC spectrum in terms of two effective energy spectral indices, αS (0.1-0.8 keV), and αH (0.4-2.4 keV), which allows for the different Galactic NH along the quasars' line of sight. We have used these data to explore the questions raised by the initial PSPC high-redshift quasar studies, and in particular the occurrence of low-energy X-ray cutoffs in high-redshift radio-loud quasars. We have also studied the emission spectra of a large sample of radio-loud and radio-quiet quasars and studied their differences. We find that low-energy X-ray cutoffs are more commonly (and perhaps exclusively) found in radio-loud quasars. Therefore, the low-energy X-ray cutoffs are physically associated with the quasars, and not with intervening systems, since those would affect radio-quiet and radio-loud equally. We suggest that photoelectric absorption is a likely origin of these cutoffs. The number of cutoffs in radio-loud quasars significantly increases with redshift rather than with luminosity. A partial correlation analysis confirms that αS is truly anticorrelated with redshift at the 99.9% confidence level, indicating evolution with cosmic epoch, and not a luminosity effect. Conversely, for αH the observed anticorrelation with redshift is mostly due to a strong dependence on luminosity. In radio-quiet quasars, we find marginal evidence for a flattening of αH (P = 4.5%), going from z 2.