IGR J16351?5806: another close by Compton-thick AGN

Malizia, A., Bassani, L., Panessa, F., De Rosa, A. and Bird, A.J. (2009) IGR J16351?5806: another close by Compton-thick AGN. Monthly Notices of the Royal Astronomical Society, 394 (1), L121-L125. (doi:10.1111/j.1745-3933.2009.00622.x).

Abstract

IGR J16351?5806 has been associated with the Seyfert 2 galaxy ESO 137?G34, having been first reported as a high energy emitter in the third INTEGRAL / Imager on Board the INTEGRAL Satellite survey. Using a new diagnostic tool based on X-ray column density measurements versus softness ratios (F2-10 keV/F20-100 keV) , we have previously identified this source as a candidate Compton-thick active galactic nuclei (AGN). In the present work, we have analysed combined XMM–Newton and INTEGRAL data of IGR J16351?5806 in order to study its broad-band spectrum and investigate its Compton-thick nature. The prominent K? fluorescence line around 6.4 keV (EW > 1 keV) together with a flat 2–10 keV spectrum immediately point to a highly obscured source. The overall spectrum can be interpreted in terms of a transmission scenario where some of the high energy radiation is able to penetrate through the thick absorption and be observed together with its reflection from the surface of the torus. A good fit is also obtained using a pure reflection spectrum; in this case, the primary continuum is totally depressed and only its reflection is observed. An alternative possibility is that of a complex absorption, where two layers of absorbing matter each partially covering the central nucleus are present in IGR J16351?5806. All three scenarios are compatible from a statistical viewpoint and provide reasonable AGN spectral parameters; more importantly all point to a source with an absorbing column greater than 1.5 × 1024 cm?2 , that is to a Compton-thick AGN. Because of this heavy obscuration, some extra components which would otherwise be hidden are able to emerge at low energies and can be studied; a thermal component with kT in the range 0.6–0.7 keV and free metal abundance is statistically required in all three scenarios while a scattered power law is only present in the pure reflection model. By providing strong evidence for the Compton-thick nature of IGR J16351?5806, we indirectly confirm the validity of the Malizia et al. diagnostic diagram.

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Contributors

Author: A.J. Bird

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