X–ray observations of the Ultraluminous infrared galaxy IRAS19254-7245 (The Superantennae)

We present ROSAT HRI and ASCA observations of the well known ULIRG IRAS19254-7245 (the Superantennae). The object is not detected by ROSAT yielding a 3σ upper limit of Lx ∼ 8× 10 erg sin the 0.1-2 keV band. However, we obtain a clear detection by ASCA yielding a luminosity in the 2-10 keV band of 2×10 erg s. Its X-ray spectrum is very hard, equivalent to a photon index of Γ = 1.0 ± 0.35. We therefore, attempt to model the X-ray data with a ”scatterer” model in which the intrinsic X-ray emission along our line of sight is obscured by an absorbing screen while some fraction, f, is scattered into our line of sight by an ionized medium; this is the standard model for the X-ray emission in obscured (but non Compton-thick) Seyfert galaxies. We obtain an absorbing column of 2 × 10 cm for a power-law photon index of Γ = 1.9, an order of magnitude above the column estimated on the basis of optical observations; the percentage of the scattered emission is high (∼ 20 per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton thick torus (NH > 10 cm) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper limit (90%) to the equivalent width of the 6.4 keV line is high (∼3 keV). All the above suggest that most of the X-ray emission originates in an highly obscured Seyfert-2 nucleus.