Volume 483, Number 2, May IV 2008
|Page(s)||415 - 424|
|Published online||11 March 2008|
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: firstname.lastname@example.org
2 Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
3 Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, UK
4 Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
Accepted: 3 March 2008
Aims. We present the results of the X-ray spectral analysis of an XMM-Newton-selected type II QSO sample with z ≥ 0.5 and 0.5-10 keV flux of 0.3-3310-14 erg/s/cm2. The distribution of absorbing column densities in type II QSOs is investigated and the dependence of absorption on X-ray luminosity and redshift is studied.
Methods. We inspected 51 spectroscopically classified type II QSO candidates from the XMM-Newton Marano field survey, the XMM-Newton-2dF wide angle survey (XWAS), and the AXIS survey to set-up a well-defined sample with secure optical type II identifications. Fourteen type II QSOs were classified and an X-ray spectral analysis performed. Since most of our sources have only ~40 X-ray counts (PN-detector), we carefully studied the fit results of the simulated X-ray spectra as a function of fit statistic and binning method. We determined that fitting the spectra with the Cash-statistic and a binning of minimum one count per bin recovers the input values of the simulated X-ray spectra best. Above 100 PN counts, the free fits of the spectrum's slope and absorbing hydrogen column density are reliable.
Results. We find only moderate absorption (N H = (2-10)1022 cm-2) and no obvious trends with redshift and intrinsic X-ray luminosity. In a few cases a Compton-thick absorber cannot be excluded. Two type II objects with no X-ray absorption were discovered. We find no evidence for an intrinsic separation between type II AGN and high X-ray luminosity type II QSO in terms of absorption. The stacked X-ray spectrum of our 14 type II QSOs shows no iron Kα line. In contrast, the stack of the 8 type II AGN reveals a very prominent iron Kα line at an energy of ~6.6 keV and an EW ~ 2 keV.
Key words: surveys / X-rays: galaxies / galaxies: active / galaxies: quasar: general
© ESO, 2008
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