On the absorption of X-ray bright broad absorption line quasars ^*,**

¹ Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
² Istituto di Astrofisica Spaziale e Fisica Cosmica (INAF), via Gobetti 101, 40129 Bologna, Italy e-mail: giustini@iasfbo.inaf.it
³ Osservatorio Astronomico di Bologna (INAF), via Ranzani 1, 40127 Bologna, Italy

Received: 10 June 2008
Accepted: 3 September 2008

Abstract

Context. Most X-ray studies of broad absorption line quasars (BALQSOs) found significant ( 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 () 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 (  10²¹ cm^-2 at 90% confidence level), and lower than typical X-ray absorption is found in the remaining sources (  10²² cm^-2) even including the faintest sources analyzed through hardness ratio analysis. The mean photon index is , with no significant evolution with redshift. The optical/X-ray spectral indices  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.