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
logL_[OIII]>9.28 L_sun from Zakamska et al. (2003). 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 NH~10^22-10^23 cm^-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 12.3
micron) 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.
