X-ray spectroscopy of the Compton-thick Seyfert 2 ESO 138 − G1

¹ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
e-mail: piconcelli@mporzio.astro.it
² Dipartimento di Fisica, Università degli Studi Roma 3, via della Vasca Navale 84, 00146 Roma, Italy
³ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Instituto de Astronomia, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 Mexico DF, Mexico

Received: 10 June 2011
Accepted: 1 August 2011

Abstract

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 (Γ ~ 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 N_H ~ 2.5 × 10²³ cm^-2 also provides a reasonable fit to the hard X-ray data, the equivalent width (EW) value of  ~800 eV measured for the Fe Kα 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 10²⁵ 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.