On the absorption of X-ray bright Broad Absorption Line Quasars

Context. Most X-ray studies of Broad Absorption Line Quasars (BALQSOs) found significant (NH ∼ 1022−24 cm−2) intrinsic column densities of gas absorbing an underlying typical power-law continuum emission, in agreement with expectations from radiatively driven accretion disk wind models. However, direct spectral analysis was performed only on a limited number of bright sources. Aims. We investigate the X-ray emission of a large number of BALQSOs at medium to high redshift (0.8 . z . 3.7) with the best data available to date. Methods. We drew a large BALQSO sample from the cross-correlation of SDSS DR5 and 2XMM catalogs to perform moderatequality X-ray spectral and hardness ratio analysis and X-ray/optical photometry. We compare our results with previous studies of BALQSOs and theoretical disk wind model expectations. Results. No or little intrinsic X-ray neutral absorption is found for one third of the spectroscopically analyzed BALQSO sample (NH < 4 × 1021 cm−2 at 90% confidence level), and lower than typical X-ray absorption is found in the remaining sources (〈NH〉 ∼ 5×1022 cm−2) even including the faintest sources analyzed through hardness ratio analysis. The mean photon index is Γ ∼ 1.9, with no significant evolution with redshift. The optical/X-ray spectral indices αox are typical of radio-quiet broad line AGN, in contrast with the known (from previous X-ray studies) “soft X-ray weakness” of BALQSOs and in agreement with the lack of X-ray absorption. We found the low-Absorption Index (AI) subsample to host the lowest X-ray absorbing column densities of the entire sample. Conclusions. X-ray selected BALQSOs show lower X-ray absorption than purely optically selected ones, and soft X-ray weakness does not hold for any of them. Their outflows may be launched by different mechanisms than classical soft X-ray weak BALQSOs or they may be the tail of the already known population seen along a different line of sight, in both cases expanding the observational parameter space for their search and investigation.