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

TL;DR

This study confirms IGR J16351-5806 as a Compton-thick AGN through broad band X-ray analysis, revealing complex absorption features and validating a diagnostic tool for identifying heavily obscured active galactic nuclei.

Contribution

The paper provides the first detailed spectral analysis confirming the Compton-thick nature of IGR J16351-5806, supporting the diagnostic method based on X-ray properties.

Findings

IGR J16351-5806 is confirmed as a Compton-thick AGN.

Spectral analysis shows complex absorption and reflection scenarios.

The diagnostic diagram for identifying Compton-thick AGN is validated.

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/IBIS survey. Using a new diagnostic tool based on X-ray column density measurements vs softness ratios, Malizia et al. (2007) identified this source as a candidate Compton thick 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_alpha 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 but a good fit is also obtained using a pure reflection spectrum. 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 x 10^24 cm^-2, i.e. 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. 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.