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.Comment: 12 pages, 12 figures, 3 tables, accepted for publication in MNRA
