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Register a new userThe quest for Type 2 quasars: Chandra observations of luminous obscured quasars in the Sloan Digital Sky Survey
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C. Vignali
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We report on new Chandra exploratory observations of six candidate Type 2 quasars at z=0.49-0.73 selected among the most [OIII] luminous emitters from the Sloan Digital Sky Survey (SDSS). Under the assumption that [OIII] is a proxy for the intrinsic luminosity of the central source, their predicted rest-frame X-ray luminosities are L(2-10keV)~10^45 erg/s. For two of the targets, the photon statistics are good enough to allow for basic X-ray spectral analyses, which indicate the presence of intrinsic absorption (~10^{22-23} cm^-2) and luminous X-ray emission (L_X>10^44 erg/s). Of the remaining four targets, two are detected with only a few (3-6) X-ray counts, and two are undetected by Chandra. If these four sources have the large intrinsic X-ray luminosities predicted by the [OIII] emission, then their nuclei must be heavily obscured (N_H>few 10^23 cm^-2) and some might be Compton thick (N_H>1.5 10^24 cm^-2). We also present the results for two Type 2 quasar candidates serendipitously lying in the fields of the Chandra targets, and provide an up-to-date compilation of the X-ray properties of eight additional SDSS Type 2 quasars from archival Chandra and XMM-Newton observations (five with moderate-quality X-ray data). The combined sample of 16 SDSS Type 2 quasars (10 X-ray detections) provides further evidence that a considerable fraction of optically selected Type 2 quasars are obscured in the X-ray band (at least all the objects with moderate-quality X-ray spectra), lending further support to the findings presented in Vignali, Alexander and Comastri (2004a) and unification schemes of Active Galactic Nuclei, and confirms the reliability of [OIII] emission in predicting the X-ray emission in obscured quasars.
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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.
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