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

We present new and archival Chandra snapshot (≈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 log L[O III] > 9.28 L⊙ from Zakamska et al. Two tar- gets 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 pres- ence of column densities in the range NH ≈ 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 ≈65 per cent of the SDSS Type 2 quasar population (LX,meas/LX,[OIII] < 0.01); however, since the Type 2 quasar sample is selected on [O III] luminosity, the estimated Compton-thick quasar fraction may be overes- timated. Using archival Spitzer observations, we find that ≈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 12.3μm) and LX,meas/LX,[OIII] ratios. We use this information to provide an estimate of the Compton-thick quasar number density at z ≈ 0.3–0.8, which we find is in broad agreement with the expectations from X-ray background models.