A Suzaku observation of the ULIRG IRAS19254-7245: discerning the AGN component

We discuss a long Suzaku observation of IRAS 19254-7245 (also known as the Superantennae), one of the brightest and well studied Ultra Luminous Infrared Galaxies in the local Universe. This long observation provided the first detection of IRAS 19254-7245 above 10 keV, and a measurement of a 15-30 keV flux of ~5× 10^-12 erg cm^-2 s^-1. The detection above 10 keV has allowed us to determine, for the first time, the intrinsic luminosity of the AGN hosted in IRAS 19254-7245, which is strongly absorbed (N_H∼ 3×10²⁴ cm^-2) and has an intrinsic luminosity (L(2-10 keV) ~3× 10⁴⁴ erg s^-1) in the QSO regime. The 2-10 keV spectrum of IRAS 19254-7245 is remarkably hard (Γ∼ 1.2), and presents a strong iron line (EW ∼ 0.7 keV), clearly suggesting that below 10 keV we observe only reprocessed radiation. Since the energy of the Fe K emission is found to be ~6.7 keV, consistent with He-like Fe, and its EW is too high to be explained by a starburst-dominated scenario, we suggest that the 2-10 keV emission of IRAS 19254-7245 is dominated by reflection/scattering from highly ionized matter. Within this latter scenario, we found that the photon index of the illuminating source is Γ=1.87^+0.11_-0.28, in excellent agreement with the mean value found for radio quiet unobscured AGN.