X-ray and optical properties of broad absorption line quasars in the Canada-France-Hawaii Telescope Legacy Survey

Abstract

We study the X-ray and optical properties of 16 Broad Absorption Line (BAL) quasars detected in about 3 degree square region common to the wide synoptic (W-1) component of the Canada-France-HawaiiTelescope Legacy Survey (CFHTLS) and the XMM Large Scale Structure survey (XMM-LSS). The BAL fraction is found to be 10% in full sample, 7% for the optical colour selected QSOs and as high as 33% if we consider QSOs selected from their IR colours. The X-ray detected non-BAL and BAL quasars have a mean observed X-ray-to-optical spectral slope of -1.47 +/- 0.13 and -1.66 +/- 0.17 respectively. We also find that the BAL QSOs have alpha_ox systematically smaller than what is expected from the relationship between optical luminosity and alpha_ox as derived from our sample. Based on this, we show, as already reported in the literature for quasars with high optical luminosities, our new sample of BAL QSOs have X-ray luminosity a factor of three smaller than what has been found for non-BAL QSOs with similar optical luminosities. Comparison of hardness ratio of the BAL and non-BAL QSOs suggests a possible soft X-ray weakness of BAL QSOs. Combining our sample, of relatively fainter QSOs, with others from the literature we show that larger balnicity index (BI) and maximum velocity (V_max) of the C IV absorption are correlated with steeper X-ray to optical spectral index. We argue that this is most likely a consequence of the existence of a lower envelope in the distribution of BI (or V_max) values versus optical luminosity. Our results thus show that the previously known X-ray weakness of BAL QSOs extends to lower optical luminosities as well.