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