The Soft X-Ray Properties of a Large Optical QSO Sample: ROSAT Observations of the Large Bright Quasar Survey

Abstract

Of the more than 1000 QSOs in the Large Bright Quasar Survey (LBQS), we study the X-ray properties of 908 that were covered by the ROSAT All-Sky Survey (RASS). These data constitute among the largest, most homogeneous 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 ≈ 600 s RASS exposure times, only 10% of the QSOs are detected in X-rays. However, by stacking X-ray counts, we obtain effectively much more sensitive observations for an average QSO in bins of redshift or luminosity, and for several classes of QSOs. We confirm a correlation of α<SUB>ox</SUB> with luminosity for the overall sample. For higher redshifts and optical luminosities, radio-loud QSOs appear to become progressively more luminous in X-rays than radio-quiet QSOs. The X-ray properties of a subsample of 36 broad absorption line QSOs suggest that they are strongly absorbed or underluminous in the X-rays, while a subsample of 22 Fe II-strong QSOs is anomalously X-ray bright.