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

We have used the WGA catalog of ROSAT PSPC X-ray sources to study the X-rayspectrum of about 500 quasars in the redshift interval 0.1--4.1, detected witha signal to noise better than 7. We have parameterized the PSPC spectrum interms of two `effective energy spectral indices', alpha_Soft (0.1-0.8 keV), andalpha_Hard (0.4-2.4 keV), which allows for the different Galactic N_H along thequasars line of sight. We have used these data to explore the questions raisedby the initial PSPC high redshift quasar studies, and in particular theoccurrence of low X-ray energy cut-offs in high redshift radio-loud quasars. Wehave also studied the emission spectra of a large sample of radio-loud andradio-quiet quasars and studied their differences. We find that low energy X-ray cut-offs are more commonly (and perhapsexclusively) found in radio-loud quasars. Therefore the low energy X-raycut-offs are physically associated with the quasars, and not with interveningsystems, since those would affect radio-quiet and radio-loud equally. Wesuggest that photoelectric absorption is a likely origin of the these cut-offs.Comment: 31 pages, LaTeX, including 6 Tables and 8 figures. Ap.J. in pres