THE SOFT-X-RAY PROPERTIES OF A LARGE OPTICAL QSO SAMPLE - ROSAT OBSERVATIONS OF THE LARGE BRIGHT QUASAR SURVEY

Of the more than 1000 QSOs in the Large Bright Quasar Survey (LBQS), w e study the X-ray properties of 908 that were covered by the ROSAT All -Sky Survey (RASS). These data constitute among the largest, most homo geneous X-ray surveys of QSOs to date, and as such are well suited to the study of the multiwavelength properties of QSOs. Due to the approx imate to 600 s RASS exposure times, only 10% of the QSOs are detected in X-rays. However, by stacking X-ray counts, we obtain effectively mu ch more sensitive observations for an average QSO in bins of redshift or luminosity, and for several classes of QSOs. We confirm a correlati on of alpha(ox) with luminosity for the overall sample. For higher red shifts and optical luminosities, radio-loud QSOs appear to become prog ressively more luminous in X-rays than radio-quiet QSOs. The X-ray pro perties of a subsample of 36 broad absorption line QSOs suggest that t hey are strongly absorbed or underluminous in the X-rays, while a subs ample of 22 Fe II-strong QSOs is anomalously X-ray bright.