EDP Sciences
Free access
Volume 504, Number 1, September II 2009
Page(s) 53 - 59
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361/200811516
Published online 09 July 2009
A&A 504, 53-59 (2009)
DOI: 10.1051/0004-6361/200811516

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

V. Braito1, 2, J. N. Reeves2, 3, R. Della Ceca4, A. Ptak2, 5, G. Risaliti6, 7, and T. Yaqoob2, 5

1  Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
    e-mail: bv67@star.le.ac.uk
2  Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
3  Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK
4  INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy
5  Astrophysics Science Division, Code 662, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
6  INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
7  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received 12 December 2008 / Accepted 27 April 2009

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 ~ $5\times
10^{-12}$ 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 ( $N_\mathrm{H}\sim 3\times10^{24}$ cm-2) and has an intrinsic luminosity (L(2-10 keV) ~ $3\times
10^{44}$ erg s-1) in the QSO regime. The 2–10 keV spectrum of IRAS 19254-7245 is remarkably hard ( $\Gamma\sim 1.2$), and presents a strong iron line ( $EW \sim 0.7$ 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 $\Gamma=1.87^{+0.11}_{-0.28}$, in excellent agreement with the mean value found for radio quiet unobscured AGN.

Key words: galaxies: active -- galaxies: individual: IRAS 19254-7245 -- galaxies: Seyfert -- X-rays: galaxies

© ESO 2009