2011A&A...534A.126P

We report on our analysis of XMM-Newton observations of the Seyfert 2 galaxy ESO 138-G1 (z=0.0091). These data reveal a complex spectrum in both its soft and hard portions. The 0.5-2keV band is characterized by a strong ``soft-excess'' component with several emission lines, as commonly observed in other narrow-line AGN. Above 3keV, a power-law fit yields a very flat slope (Γ∼0.35), along with the presence of a prominent line-like emission feature around ∼6.4keV. This indicates heavy obscuration along the line of sight to the nucleus. We find an excellent fit to the 3-10keV continuum with a pure reflection model, which provides strong evidence of a Compton-thick screen, preventing direct detection of the intrinsic nuclear X-ray emission. Although a model consisting of a power law transmitted through an absorber with N_H∼2.5x10²³cm^–2 also provides a reasonable fit to the hard X-ray data, the equivalent width (EW) value of ∼800eV measured for the FeKα emission line is inconsistent with a primary continuum obscured by a Compton-thin column density. Furthermore, the ratio of 2-10keV to de-reddened [OIII] fluxes for ESO 138-G1 agrees with the typical values reported for well-studied Compton-thick Seyfert galaxies. Finally, we also note that the upper limits to the 15-150keV flux provided by Swift/BAT and INTEGRAL/IBIS seem to rule out the presence of a transmitted component of the nuclear continuum even in this very hard X-ray band, hence imply that the column density of the absorber could be as high as 10²⁵cm^–2. This makes ESO 138-G1 a very interesting, heavy Compton-thick AGN candidate for the next X-ray missions with spectroscopic and imaging capabilities above 10keV.