Evidence for X-ray obscuration in Type II quasar candidates from the Sloan Digital Sky Survey

Abstract

Recently, Zakamska et al. selected 291 high-ionization narrow-emission-line active galactic nuclei (AGN) in the redshift range 0.3-0.83 from the Sloan Digital Sky Survey (SDSS) spectroscopic data. The sample includes both type 2 Seyfert 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 (>~6σ) 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 ~6σ detection in the ROSAT All-Sky Survey (RASS), 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 N_H= 1.26^+0.75_-0.51× 10²² cm^-2 this result, coupled with the 2-10 keV luminosity of ~ 4 × 10⁴⁴ erg s^-1, makes this source a genuine Type II quasar. Using the [OIII]λ5007 line luminosities, we estimated the intrinsic X-ray power of the AGN and found that >~47 per cent of the observed sample shows indications of X-ray absorption, with column densities >~10²² cm^-2. This provides further evidence that a considerable fraction are obscured quasars. Support for our conclusions also comes from the very recent analysis of RASS data performed by Zakamska et al., who found five additional lower-significance (~2.1σ-3.6σ) X-ray matches.