Broad absorption line quasars ( commonly termed BALQSOs ) contain the most dramatic examples of AGN-driven winds .
The high absorbing columns in these winds , \sim 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 \sim 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 } can not 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 .