A Chandra observation of the ultraluminous infrared galaxy IRAS 19254--7245 (the Superantennae): X-ray emission from the Compton-thick AGN and the diffuse starburst

Abstract

We present a {\it Chandra} observation of IRAS 19254--7245, a nearby ULIRG also known as {\it the Superantennae}. The high spatial resolution of {\it Chandra} allows us to disentangle for the first time the diffuse starburst emission from the embedded Compton-thick AGN. The 2-10 keV spectrum of the AGN emission is fitted by a flat power-law $\Gamma=1.3$) and a He-like Fe K$\alpha$ line with EW$\sim$1.5 keV, consistent with previous observations. The Fe K$\alpha$ line profile could be resolved as a blend of a neutral 6.4 keV line and an ionized 6.7 keV (He-like) or 6.9 keV (H-like) line. Variability is detected compared with the previous {\it XMM-Newton} and {\it suzaku} observations, demonstrating the compact size of the iron line emission. We fit the spectrum of the galaxy-scale extended emission excluding the AGN and other bright point sources with a soft thermal component with kT~0.8 keV. The luminosity of the extended emission is about one order of magnitude lower than that of the AGN. The basic physical and structural properties of the extended emission are fully consistent with a galactic wind being driven by the starburst (no contribution to the feedback by the AGN is required). A candidate ultra-luminous X-ray source is detected 8\arcsec\ south of the southern nucleus. The 0.3-10 keV luminosity of this off-nuclear point source is ~$6\times 10^{40}$ erg s$^{-1}$ if the emission is isotropic and the source is associated with the Superantennae.