The quest for Type 2 quasars: Chandra observations of luminous obscured quasars in the Sloan Digital Sky Survey

Abstract

We report on new Chandra exploratory observations of six candidate Type 2 quasars at z = 0.49-0.73 selected among the most [O III] luminous emitters from the Sloan Digital Sky Survey (SDSS). Under the assumption that [O III] is a proxy for the intrinsic luminosity of the central source, their predicted rest-frame X-ray luminosities are L 2-10 keV ≈ 10 45 erg s -1 . 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 ? 1044 erg s -1 ). 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 [O III] emission, then their nuclei must be heavily obscured (N H > few times 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 by Vignali, Alexander & Comastri and unification schemes of active galactic nuclei, and confirms the reliability of [O III] emission in predicting the X-ray emission in obscured quasars.