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

• Published in: The Astrophysical journal Vol. 859; no. 2; pp. 113 - 123
• Main Authors: Liu, Hezhen, Luo, B., Brandt, W. N., Gallagher, S. C., Garmire, G. P.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.06.2018; IOP Publishing
• Subjects: Absorption; Active galactic nuclei; Astrophysics; galaxies: active; Ionization; Polls & surveys; Power law; Quasars; quasars: absorption lines; Target detection; Ultraviolet spectra; X-rays: general
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aabe8d

We present combined 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 ( and ) and significant X-ray weakness in the hard band (by factors of 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 7%-10% (2/29-3/29), and we estimate it is 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.