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|Title:||The XMM-SSC survey of hard-spectrum XMM-Newton sources - I. Optically bright sources|
Watson, Michael G.
Della Ceca, R.
|Publisher:||Blackwell Publishing on behalf of the Royal Astronomical Society|
|Citation:||Monthly Notices of the Royal Astronomical Society, 2007, 378 (4), pp. 1335-1355 (21)|
|Abstract:||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.|
|Rights:||© 2007 The Authors. Journal compilation © 2007 RAS. Deposited with reference to the publisher's archiving policy available from the Journal Exclusive Licence Form, which permits posting of the Blackwell Publishing PDF 6 months after publication. The definitive version is available at www.blackwell-synergy.com|
|Appears in Collections:||Published Articles, Dept. of Physics and Astronomy|
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|j.1365-2966.2007.11857.x.pdf||Publisher Version||1.6 MB||Adobe PDF||View/Open|
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