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Description
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 ({alpha}_OX_) and the 2500{AA} monochromatic luminosity (L_2500{AA}_), and the X-ray weakness of a quasar is assessed via the deviation of its {alpha}_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; CIV rest-frame equivalent width <16{AA}), red quasars ({Delta}(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.
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