Astronomy & Astrophysics

Table of contents

A&A 483, 415-424 (2008)
DOI: 10.1051/0004-6361:20079012

X-ray absorption in distant type II QSOs

M. Krumpe¹, G. Lamer¹, A. Corral², A. D. Schwope¹, F. J. Carrera², X. Barcons², M. Page³, S. Mateos⁴, J. A. Tedds⁴, and M. G. Watson⁴

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 $\ge$ 0.5 and 0.5-10 keV flux of 0.3-33 $\times$ 10^-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 ( $N_{\rm H}$ = (2-10) $\times$ 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 K $\alpha$ line. In contrast, the stack of the 8 type II AGN reveals a very prominent iron K $\alpha$ line at an energy of ~6.6 keV and an EW ~ 2 keV.

Key words: surveys -- X-rays: galaxies -- galaxies: active -- galaxies: quasar: general