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 ($\hat{I}{\pm }_{\mathrm{OX}}$) and the 2500 $\tilde{A}$ monochromatic luminosity ($L_{2500\tilde{\mathrm{A}}}$), and the X-ray weakness of a quasar is assessed via the deviation of its $\hat{I}{\pm }_{\mathrm{OX}}$ 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 $\ge 6$ and $\ge 10$ are $5.8\pm 0.7\%$ and $2.7\pm 0.5\%$, respectively. We classify the X-ray weak quasars (X-ray weak by factors of $\ge 6$) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; CIV REW $<16\tilde{A}$), red quasars ($\hat{I}”(g-i)>0.2$), 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.

24 pages, 10 figures, accepted for publication in ApJ