University of Leicester
j.1365-2966.2007.11857.x.pdf (1.56 MB)

The XMM-SSC survey of hard-spectrum XMM-Newton sources - I. Optically bright sources

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posted on 2012-09-25, 14:58 authored by M.J. Page, I. Lehmann, T. Boller, Michael G. Watson, T. Dwelly, S. Hess, I. Matute, N.S. Loaring, S. Rosen, H. Ziaeepour, A. Schwope, G. Lamer, F.J. Carrera, J. Tedds, R. Della Ceca, P. Severgnini, R.G. McMahon, W. Yuan
We present optical and X-ray data for a sample of serendipitous XMM-Newton sources that are selected to have 0.5-2 versus 2-4.5 keV X-ray hardness ratios which are harder than the X-ray background. The sources have 2-4.5 keV X-ray flux , and in this paper we examine a subsample of 42 optically bright (r < 21)> sources; this subsample is 100 per cent spectroscopically identified. All but one of the optical counterparts are extragalactic, and we argue that the single exception, a Galactic M star, is probably a coincidental association rather than the correct identification of the X-ray source. The X-ray spectra of all the sources are consistent with heavily absorbed power laws (21.8 < log N(H) < 23.4), and all of them, including the two sources with 2-10 keV intrinsic luminosities of < 10(42) erg s(-1), appear to be absorbed active galactic nuclei (AGN). The majority of the sources show only narrow emission lines in their optical spectra, implying that they are type 2 AGN. Three sources have 2-10 keV luminosities of > 10(44) erg s(-1), and two of these sources have optical spectra which are dominated by narrow emission lines, that is, are type 2 QSOs. Only a small fraction of the sources (7/42) show broad optical emission lines, and all of these have N(H) < 10(23) cm(-2). This implies that ratios of X-ray absorption to optical/ultraviolet extinction equivalent to > 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a few per cent of the population), and may be restricted to broad absorption line QSOs. Seven objects appear to have an additional soft X-ray component in addition to the heavily absorbed power law; all seven are narrow emission-line objects with z < 0.3 and 2-10 keV intrinsic luminosities < 10(43) erg s(-1). We consider the implications of our results in the light of the AGN unified scheme. We find that the soft components in narrow-line objects are consistent with the unified scheme provided that > 4 per cent of broad-line AGN (BLAGN) have ionized absorbers that attenuate their soft X-ray flux by > 50 per cent. In at least one of the X-ray-absorbed BLAGN in our sample the X-ray spectrum requires an ionized absorber, consistent with this picture.



Monthly Notices of the Royal Astronomical Society, 2007, 378 (4), pp. 1335-1355 (21)

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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