X-ray spectroscopy of the Compton-thick Seyfert 2 ESO 138-G1
  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 0.5-2 keV band is characterized by a strong
'soft-excess' component with several emission lines, as commonly observed in
other narrow-line AGN. Above 3 keV, a power-law fit yields a very flat slope
(Gamma ~0.35), along with the presence of a prominent line-like emission
feature around ~6.4 keV. This indicates heavy obscuration along the line of
sight to the nucleus. We find an excellent fit to the 3-10 keV 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
Nh ~2.5 x 10^{23} cm^-2 also provides a reasonable fit to the hard X-ray data,
the equivalent width value of ~800 eV measured for the Fe Kalpha emission line
is inconsistent with a primary continuum obscured by a Compton-thin column
density. Furthermore, the ratio of 2-10 keV 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 15-150 keV 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 10^{25} cm^-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 10 keV.
