BROAD ABSORPTION-LINE QUASARS OBSERVED BY THE ROSAT-PSPC

Recent results from the ROSAT All-Sky Survey have shown that broad abs orption line (BAL) QSOs are either highly absorbed or underluminous in the soft X-ray bandpass. Here we extend this work by analyzing all kn own bona fide BAL QSOs observed within the inner 20' of the ROSAT Posi tion Sensitive Proportional Counter. This sample includes both targete d and serendipitous exposures ranging from 8 to 75 ks. Despite these d eep exposures, most of the BAL QSOs are undetected and have unusually weak X-ray emission, as evidenced by large optical-to-X-ray slopes alp ha(ox). Large values of alpha(ox) (greater than or similar to 1.8) may prove to be a defining characteristic of BAL QSOs. We predict that sa mples of QSO candidates with large alpha(ox) will yield a higher perce ntage of BAL QSOs, particularly at low redshift. As a corollary, X-ray -selected QSO samples should yield fewer BAL QSOs. The optical/UV emis sion line spectra of BAL and non-BAL QSOs are quite similar, suggestin g that their intrinsic spectral energy distributions are similar as we ll. Absorption thus seems the likely reason for the X-ray-quiet nature of BAL QSOs. To constrain the total absorbing column of the BAL cloud s, we compare our measured soft X-ray fluxes or upper limits with thos e expected from normal radio-quiet QSOs of comparable optical continuu m magnitude and redshift. From sensitive X-ray observations, we derive column densities of greater than or similar to 2 x 10(22) Cm-2 for in trinsic cold absorbers of solar metallicity. These new results suggest columns at least an order of magnitude larger than the columns previo usly estimated from optical/UV spectra alone.