ROSAT SOFT-X-RAY PROPERTIES OF THE LARGE BRIGHT QUASAR SURVEY - MODELING OF STACKED X-RAY-SPECTRA

We develop and apply a novel method of analysis to study the X-ray spe ctral properties of 908 QSOs in the Large Bright Quasar Survey (LBQS) that were observed during the soft X-ray ROSAT All-Sky Survey (RASS), Due to the relatively short (less than or equal to 600 s) RASS exposur e times, only 10 per cent of the QSOs are detected in X-rays, so X-ray spectral model fits for individual QSOs are precluded by poor photon statistics. Spectral stacking provides effectively much more sensitive X-ray observations for an average QSO in bins of redshift, and for se veral classes of QSOs. We model the stacked X-ray spectra in st way th at obviates both the distinction between detections and non-detections , and the need to stack together only those objects observed through s imilar Galactic column densities N-H. In application to the LBQS/RASS sample, we marginally confirm a flattening of the X-ray spectral index of QSOs toward higher redshifts. Radio-loud QSOs show flatter X-ray p hoton indices, in agreement with previous work. We show that the appar ent flattening of the photon index with redshift is not due to an incr easing fractional contribution from the radio-loud class of QSOs.