paper

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

arXiv:0809.0832 · doi:10.1051/0004-6361:200810363

Abstract

Most X-ray studies of 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. We investigate the X-ray emission of a large BALQSO sample at medium to high redshift (0.8<z<3.7), drawn from the cross-correlation of SDSS DR5 and 2XMM catalogs. We perform on it moderate-quality X-ray spectral and hardness ratio analysis, and X-ray/optical photometry. No or little intrinsic X-ray neutral absorption is found for one third of the spectroscopically analyzed BALQSO sample (N_H < 4 x 10^{21} cm^{-2} at 90% confidence level), and lower than typical X-ray absorption is found in the remaining sources (<N_H> ~ 5 x 10^{22} cm^{-2}) even including the faintest sources analyzed through hardness ratio analysis. The mean photon index is Gamma~1.9, with no significant evolution with redshift. The alpha_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.

arXiv commentAccepted by A&A. 20 pages, 10 figures, 5 tables. Appendices available in online edition. Typos corrected, two references added, improved figures

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