X-ray-selected broad absorption line quasi-stellar objects

We study a sample of six X-ray-selected broad absorption line (BAL) quasi-stellar objects (QSOs) from the XMM–Newton Wide Angle Survey. All six objects are classified as BALQSOs using the classic balnicity index, and together they form the largest sample of X-ray-selected BALQSOs. We find evidence for absorption in the X-ray spectra of all six objects. An ionized absorption model applied to an X-ray spectral shape that would be typical for non-BAL QSOs (a power law with energy index α = 0.98) provides acceptable fits to the X-ray spectra of all six objects. The optical to X-ray spectral indices, αOX, of the X-ray-selected BALQSOs, have a mean value of 〈αOX〉 = 1.69 ± 0.05, which is similar to that found for X-ray-selected and optically selected non-BAL QSOs of a similar ultraviolet luminosity. In contrast, optically selected BALQSOs typically have much larger αOX and so are characterized as being X-ray weak. The results imply that X-ray selection yields intrinsically X-ray bright BALQSOs, but their X-ray spectra are absorbed by a similar degree to that seen in optically selected BALQSO samples; X-ray absorption appears to be ubiquitous in BALQSOs, but X-ray weakness is not. We argue that BALQSOs sit at one end of a spectrum of X-ray absorption properties in QSOs related to the degree of ultraviolet absorption in C IV 1550 Å. ; This research was based on observations obtained with XMM– Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the National Aeronautics and Space Administration. This research was also based on observations made at the Anglo-Australian Telescope. MJP acknowledges financial support from the UK Science and Technology Facilities Council. FJC and SM acknowledge financial support through grant AYA2015-64346-C2-1-P (MINECO/FEDER). MTC acknowledges support by the Spanish Programma Nacional de Astronomia y Astrofisica under grant AYA2009-08059. MK acknowledges support by DFG grant KR 3338/3-1. The NASA/IPAC Extragalactic Database is operated by ...