The Reddest DR3 SDSS/XMM-Newton Quasars

Abstract

We have cross-correlated the SDSS DR3 Schneider et al. (2005) quasar catalog with the XMM-Newton archive. Color and redshift selections (g - r >= 0.5 and 0.9 < z < 2.1) result in a sample of 17 red, moderate-redshift quasars. The redshift selection minimizes possible contamination due to host galaxy emission and Lyα forest absorption. Both optical and X-ray information is required to distinguish between the two likely remaining causes of the red colors: (1) dust reddening and (2) an intrinsically red continuum. We find that 7 of 17 quasars can be classified as probable "intrinsically red" objects. These seven quasars have unusually broad Mg II emission lines (langleFWHMrangle = 10,500 km s^-1), moderately flat but unabsorbed X-ray spectra (langleΓrangle = 1.66 +/- 0.08), and low accretion rates ( dot M/dot M_Edd ~ 0.01). We suggest low accretion rates as a possible physical explanation for quasars with intrinsically red optical continua. We find that 8 of 17 quasars can be classified as dust reddened. Three of these have upper limits on the absorption column from X-ray spectral fits of N_H = (3-13) × 10²² cm², while the other five quasars must be absorbed by at least N_H = 10²³ cm² in order to be consistent with a comparably selected α_ox-l_uv distribution. Two objects in the sample are unclassified.