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

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.

Accepted 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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Cited by in corpus (9)

• Compton Thick AGN in the XMM-COSMOS survey
• Weak Hard X-ray Emission from Broad Absorption Line Quasars: Evidence for Intrinsic X-ray Weakness
• Widespread, strong outflows in XQR-30 quasars at the Reionisation epoch
• Radiation pressure confinement - IV. Application to broad absorption line outflows
• The fraction and kinematics of broad absorption line quasars across cosmic time
• What controls the UV-to-X-ray continuum shape in quasars?
• Chandra X-Ray Observations of Two Unusual BAL Quasars
• Coordinated X-ray and UV absorption within the accretion disk wind of the active galactic nucleus PG 1126-041
• X-ray selected broad absorption line quasars in SDSS-V: BALs and non-BALs span the same range of X-ray properties