On the Fraction of X-ray Weak Quasars from the Sloan Digital Sky Survey

Abstract

We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey (SDSS) 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 2500 Å), 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 the 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 REW < 16 Å), red quasars (∆(g − i) > 0.2), and unclassified X-ray weak quasars. The X-ray weak fraction of 35 +12 −9 % 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 +5 −3 % 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.