Evidence for X-ray Obscuration in Type II Quasar Candidates from the Sloan Digital Sky Survey

Abstract

Recently, Zakamska et al. (2003) selected 291 high-ionization narrow emission-line AGN in the redshift range 0.3-0.83 from the Sloan Digital Sky Survey spectroscopic data. The sample includes both Seyfert II galaxies and their higher luminosity "cousins", Type II quasar candidates. Here we present the results on the X-ray properties of 17 of these objects for which archival X-ray data (ROSAT and XMM-Newton) are available. Three sources have been significantly (>6 sigma) detected, one being the most radio-loud source of the sample; its X-ray emission, possibly enhanced by jet emission, is consistent with the absence of absorption. Another source has a ~6 sigma detection in the ROSAT All-Sky Survey, possibly complex radio structure, and no evidence for strong X-ray absorption. For the third X-ray detection, the XMM-Newton spectrum indicates a column density of NH=1.26+0.75/-0.51x10^22 cm^-2; this result, coupled with the 2-10 keV luminosity of ~4x10^44 cgs, makes this source a genuine Type II quasar. Using the O[III]5007 line luminosities, we estimated the intrinsic X-ray power of the AGN and found that >47% of the observed sample shows indications of X-ray absorption, with column densities >10^22 cm^-2. This provides further evidence that a considerable fraction are obscured quasars. Support to our conclusions also comes from the recent analysis of RASS data performed by Zakamska et al. (2004), who found five additional lower significance (~2.1-3.6 sigma) X-ray matches.