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

• Published in: Monthly notices of the Royal Astronomical Society 2007-07, Vol.378 (4), p.1335-1355
• Main Authors: Page, M. J., Lehmann, I., Boller, Th, Watson, M. G., Dwelly, T., Hess, S., Matute, I., Loaring, N. S., Rosen, S., Ziaeepour, H., Schwope, A., Lamer, G., Carrera, F. J., Tedds, J., Della Ceca, R., Severgnini, P., McMahon, R. G., Yuan, W.
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics; Earth, ocean, space; Exact sciences and technology; galaxies: active; Optics; Stars & galaxies; X-ray astronomy; X-rays: galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2007.11857.x

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 ≥10−14  erg cm−2  s−1, 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  NH < 23.4), and all of them, including the two sources with 2–10 keV intrinsic luminosities of 1044  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 NH < 1023  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 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.