posted on 2012-10-24, 09:22authored byM. Krumpe, G. Lamer, A. D. Schwope, A. Corral, F. J. Carrera, X. Barcons, M. Page, S. Mateos, Jonathan A. Tedds, M. G. Watson
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.
History
Citation
Astronomy & Astrophysics, 2008, 483 (2), pp. 415-424
Version
VoR (Version of Record)
Published in
Astronomy & Astrophysics
Publisher
EDP Sciences for European Southern Observatory (ESO)