Properties of X-ray-selected broad absorption-line quasars

Broad absorption-line quasars (commonly termed BALQSOs) contain the most dramatic examples of active galactic nucleus (AGN) driven winds. The high absorbing columns in these winds, ∼10²⁴ cm^-2, ensure that BALQSOs are generally X-ray faint. This high X-ray absorption means that almost all BALQSOs have been discovered through optical surveys, and so what little we know about their X-ray properties is derived from very bright optically selected sources. A small number of X-ray-selected BALQSOs (XBALQSOs) have, however, recently been found in deep X-ray survey fields. In this paper we investigate the X-ray and rest-frame ultraviolet (UV) properties of five XBALQSOs for which we have obtained XMM-Newton EPIC X-ray spectra and deep optical imaging and spectroscopy. We find that, although the XBALQSOs have an α_ox steeper by ∼0.5 than normal QSOs, their median α_ox is nevertheless flatter by 0.30 than that of a comparable sample of optically selected BALQSOs (OBALQSOs). We rule out the possibility that the higher X-ray to optical flux ratio is due to intrinsic optical extinction. We find that the amount of X-ray and UV absorption due to the wind in XBALQSOs is similar, or perhaps greater than, the corresponding wind absorption in OBALQSOs, so the flatter α_ox cannot be a result of weaker wind absorption. We conclude that these XBALQSOs have intrinsically higher X-ray to optical flux ratios than the OBALQSO sample with which we compare them.