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X-ray absorption in distant type II QSOs

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journal contribution
posted on 24.10.2012, 09:22 by M. 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.



Astronomy & Astrophysics, 2008, 483 (2), pp. 415-424


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