2008A&A...491..425G

Most X-ray studies of broad absorption line quasars (BALQSOs) found significant (N_H∼10^22–24cm^–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. 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. We drew a large BALQSO sample from the cross-correlation of SDSS DR5 and 2XMM catalogs to perform moderate-quality 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. No or little intrinsic X-ray neutral absorption is found for one third of the spectroscopically analyzed BALQSO sample (N_H<4x10²¹cm^–2 at 90% confidence level), and lower than typical X-ray absorption is found in the remaining sources (<N_H≳5x10²²cm^–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 α_ox 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. 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.