Broad Absorption Line Qsos Observed by the Rosat Pspc

Recent results from the ROSAT All-Sky Survey (RASS) have shown that broad absorption line (BAL) QSOs are either highly absorbed or underluminous in the soft X-ray bandpass. Here, we extend this work by analyzing all known bona fide BALQSOs observed within the inner 20 of the ROSAT PSPC. This sample includes both targeted and serendipitous exposures ranging from 8 to 75 ksec. Despite these deep exposures, most of the BALQSOs are undetected, and have unusually weak X-ray emission, as evidenced by large optical-to-X-ray slopes αox. Large values of αox ( > ∼ 1.8) may prove to be a defining characteristic of BALQSOs. We predict that samples of QSO candidates with large αox will yield a higher percentage of BALQSOs, particularly at low redshift. As a corollary, X-ray-selected QSO samples should yield fewer BALQSOs. The optical/UV emission line spectra of BAL and non-BAL QSOs are quite similar, suggesting that their intrinsic spectral energy distributions are similar as well. Absorption thus seems the likely reason for the X-ray quiet nature of BALQSOs. To constrain the total absorbing column of the BAL clouds, we compare our measured soft X-ray fluxes or upper limits to those expected from normal radio quiet QSOs of comparable optical continuum magnitude and redshift. From sensitive X-ray observations, we derive column densities of > ∼ 2×10 cm for intrinsic cold absorbers of solar metallicity. These new results suggest columns at least an order of magnitude larger than the columns previously estimated from optical/UV spectra alone. Subject headings: galaxies: active — quasars: general — X-rays: galaxies Hubble Fellow [email protected]