X-ray absorption in distant type II QSOs ^*,**

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: mkrumpe@aip.de
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

Received: 6 November 2007
Accepted: 3 March 2008

Abstract

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.