Open AccessEvidence for X‐ray obscuration in Type II quasar candidates from the Sloan Digital Sky Survey
Author(s) -
Vignali Cristian,
Alexander Dave M.,
Comastri Andrea
Publication year - 2004
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2004.08231.x
Subject(s) - physics , astrophysics , rosat , quasar , redshift , sky , active galactic nucleus , luminosity , astronomy , galaxy , emission spectrum , qsos , doubly ionized oxygen , line (geometry) , spectral line , geometry , mathematics
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 × 10 22 cm −2 ; this result, coupled with the 2–10 keV luminosity of ≈ 4 × 10 44 erg s −1 , makes this source a genuine Type II quasar. Using the [O iii ]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 ≳10 22 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.