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

Abstract

We present new and archival Chandra snapshot (~10ks 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_[OIII] > 9.28L_solar 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 0.5-8keV 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 ~ 10²²-10²³cm^-2 in the source rest frame. If the [OIII] 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 (L_X,meas/L_X,[OIII] < 0.01); however, since the Type 2 quasar sample is selected on [OIII] luminosity, the estimated Compton-thick quasar fraction may be overestimated. 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 L_X,meas/L_X,mid-IR (where mid-IR corresponds to rest frame 12.3μm) and L_X,meas/L_X,[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.