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

Author： Vignali C. Alexander D. M. Gilli R. Pozzi F.

ISSN： 0035-8711

Source： Monthly Notices of the Royal Astronomical Society, Vol.404, Iss.1, 2010-05, pp. : 48-59

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Abstract

ABSTRACTWe 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 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≈ 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 (L_X,meas/L_{X,[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 ≈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 μm) and L_X,meas/L_{X,[O III]} 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.