The Frequency of Intrinsic X-Ray Weakness among Broad Absorption Line Quasars

We present combined approximate to 14-37 ks Chandra observations of seven z = 1.6-2.7 broad absorption line (BAL) quasars selected from the Large Bright Quasar Survey (LBQS). These seven objects are high-ionization BAL (HiBAL) quasars, and they were undetected in the Chandra hard band (2-8 keV) in previous observations. The stacking analyses of previous Chandra observations suggested that these seven objects likely contain some candidates for intrinsically X-ray weak BAL quasars. With the new Chandra observations, six targets are detected. We calculate their effective power-law photon indices and hard-band flux weakness, and find that two objects, LBQS 1203 + 1530 and LBQS 1442-0011, show soft/steep spectral shapes (Gamma(eff) = 2.2(-0.9)(+0.9) and 1.9(-0.8)(+0.9) and significant X-ray weakness in the hard band (by factors of approximate to 15 and 12). We conclude that the two HiBAL quasars are good candidates for intrinsically X-ray weak BAL quasars. The mid-infrared-to-ultraviolet spectral energy distributions of the two candidates are consistent with those of typical quasars. We constrain the fraction of intrinsically X-ray weak active galactic nuclei (AGNs) among HiBAL quasars to be approximate to 7%-10% (2/29-3/29), and we estimate it is approximate to 6%-23% (2/35-8/35) among the general BAL quasar population. Such a fraction is considerably larger than that among non-BAL quasars, and we suggest that intrinsically X-ray weak quasars are preferentially observed as BAL quasars. Intrinsically X-ray weak AGNs likely comprise a small minority of the luminous type. 1 AGN population, and they should not affect significantly the completeness of these AGNs found in deep X-ray surveys.