X-ray spectral survey of WGACAT quasars. I. Spectral evolution and low-energy cutoffs

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," alpha(s) (0.1-0.8 keV), and alpha(H) (0.4-2.4 keV), which allows for the different Galactic N-H 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 alpha(S) is truly anticorrelated with redshift at the 99.9% confidence level, indicating evolution with cosmic epoch, and not a luminosity effect. Conversely, for alpha(H) 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 alpha(H) (P = 4.5%), going from z < 1 to z = 2; in agreement with previous studies. Radio-loud quasars at z < 2.2 instead show a "concave" spectrum (alpha(H) < alpha(S) by similar to 0.2). This new result is consistent with the widespread suggestion that the flatter X-ray spectra of radio-loud quasars may be due to an additional component above those seen in radio-quiet quasars. However, it might also imply different processes at work in radio-loud and radio-quiet sources. At z greater than or similar to 2 the average soft and hard indices are similar and are both significantly smaller than at lower redshifts. This can be due to the soft component of radio-loud quasars being completely shifted out of the PSPC band at z > 2.