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 NH= 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2004
- DOI:
- 10.1111/j.1365-2966.2004.08231.x
- arXiv:
- arXiv:astro-ph/0407293
- Bibcode:
- 2004MNRAS.354..720V
- Keywords:
-
- galaxies: active;
- galaxies: nuclei;
- quasars: general;
- X-rays: galaxies;
- Astrophysics
- E-Print:
- 7 pages, 3 figures, 2 tables, accepted for publication in MNRAS