Discovery of Compton-thick quasars in the Sloan Digital Sky Survey

We present new and archival Chandra snapshot (approximate to 10 ks each) observations of 15 optically identified [from the Sloan Digital Sky Survey (SDSS)] Type 2 quasars at z = 0.40-0.73. When combined with existing X-ray data, this work provides complete X-ray coverage for all 25 radio-quiet Type 2 quasars with logL([O III]) > 9.28 L-circle dot from Zakamska et al. Two targets out of 15 were not detected by Chandra and most of the remaining sources are X-ray weak, with nine having less than 10 counts in the 0.5-8 keV band. Low-to moderate-quality spectral analysis was limited to three sources, whose properties are consistent with the presence of column densities in the range N-H approximate to 1022-1023 cm(-2) in the source rest frame. If the [O III] luminosity is a reliable proxy for the intrinsic X-ray luminosity, the current X-ray data indicate that Compton-thick quasars may hide among approximate to 65 per cent of the SDSS Type 2 quasar population (L-X,L-meas/L-X,L-[O III] < 0.01); however, since the Type 2 quasar sample is selected on [O III] luminosity, the estimated Compton- thick quasar fraction may be overestimated. Using archival Spitzer observations, we find that approximate to 50 per cent of SDSS Type 2 quasars appear to be obscured by Compton- thick material based on both the LX, meas/LX, mid-IR (where mid-IR corresponds to rest frame 12.3 mu m) and LX, meas/LX,[O III] ratios. We use this information to provide an estimate of the Compton- thick quasar number density at z approximate to 0.3-0.8, which we find is in broad agreement with the expectations from X-ray background models.