On the Fraction of X-Ray-weak Quasars from the Sloan Digital Sky Survey

We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey non-broad absorption line (non-BAL) quasars with Chandra archival observations. A significant correlation is found between the X-ray-to-optical power-law slope parameter (α OX) and the 2500 Å monochromatic luminosity (L 2500Å), and the X-ray weakness of a quasar is assessed via the deviation of its α OX value from that expected from this relation. We demonstrate the existence of a population of non-BAL X-ray-weak quasars, and the fractions of quasars that are X-ray weak by factors of ≥6 and ≥10 are 5.8% ± 0.7% and 2.7% ± 0.5%, respectively. We classify X-ray-weak quasars (X-ray weak by factors of ≥6) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; C iv rest-frame equivalent width < 16 Å), red quasars (Δ(g - i) > 0.2), and unclassified X-ray-weak quasars. The X-ray-weak fraction of 35-9+12% within the WLQ population is significantly higher than that within non-WLQs, confirming previous findings that WLQs represent one population of X-ray-weak quasars. The X-ray-weak fraction of 13-3+5% within the red quasar population is also considerably higher than that within the normal quasar population. The unclassified X-ray-weak quasars do not have unusual optical spectral features, and their X-ray weakness may be mainly related to quasar X-ray variability.