Properties of X-ray selected Broad Absorption Line Quasars

TL;DR

This study investigates the X-ray and UV properties of five X-ray selected BALQSOs, revealing they have higher intrinsic X-ray to optical flux ratios than optically selected BALQSOs, challenging previous assumptions about their absorption characteristics.

Contribution

First detailed analysis of X-ray selected BALQSOs showing they have higher intrinsic X-ray to optical flux ratios compared to optically selected samples.

Findings

X-ray selected BALQSOs have flatter alpha_ox than optically selected BALQSOs.

X-ray and UV absorption levels are similar or greater in XBALQSOs.

Intrinsic X-ray to optical flux ratio is higher in XBALQSOs.

Abstract

Broad absorption line quasars (commonly termed BALQSOs) contain the most dramatic examples of AGN-driven winds. The high absorbing columns in these winds, ~10^24 cm^-2, ensure that BALQSOs are generally X-ray faint. This high X-ray absorption means that almost all BALQSOs have been discovered through optical surveys, and so what little we know about their X-ray properties is derived from very bright optically-selected sources. A small number of X-ray selected BALQSOs (XBALQSOs) have, however, recently been found in deep X-ray survey fields. In this paper we investigate the X-ray and rest-frame UV properties of five XBALQSOs for which we have obtained XMM-Newton EPIC X-ray spectra and deep optical imaging and spectroscopy. We find that, although the XBALQSOs have an alpha_ox steeper by ~0.5 than normal QSOs, their median alpha_ox is nevertheless flatter by 0.30 than that of a comparable sample of optically selected BALQSOs (OBALQSOs). We rule out the possibility that the higher X-ray to optical flux ratio is due to intrinsic optical extinction. We find that the amount of X-ray and UV absorption due to the wind in XBALQSOs is similar, or perhaps greater than, the corresponding wind absorption in OBALQSOs, so the flatter alpha_ox cannot be a result of weaker wind absorption. We conclude that these XBALQSOs have intrinsically higher X-ray to optical flux ratios than the OBALQSO sample with which we compare them.