On the absorption of X-ray bright broad absorption line quasars

Abstract

Context: Most X-ray studies of broad absorption line quasars (BALQSOs) found significant (N_H∼ 10^22-24 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 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.
Results: No or little intrinsic X-ray neutral absorption is found for one third of the spectroscopically analyzed BALQSO sample (N_H < 4 × 10²¹ cm^-2 at 90% confidence level), and lower than typical X-ray absorption is found in the remaining sources (< N_H> ∼ 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 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.
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.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Appendices A and B are only available in electronic form at http://www.aanda.org