2000MNRAS.314..589P

We present ROSAT High Resolution Imager (HRI) and ASCA observations of the well-known ultraluminous infrared galaxy (ULIRG) IRAS 19254-7245 (the `Superantennae'). The object is not detected by ROSAT, implying a 3σ upper limit of X-ray luminosity L<sub>X</sub>∼8x10<sup>41</sup>erg.s<sup>–1</sup> in the 0.1-2keV band. However, we obtain a clear detection by ASCA, yielding a luminosity in the 2-10keV band of 2x10<sup>42</sup>erg.s<sup>–1</sup>. The X-ray spectrum of IRAS 19254-7245 is very hard, equivalent to a photon index of Γ=1.0±0.35. We therefore attempt to model the X-ray data using 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 density of N_H=2x10²³cm^–2 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 (∼20per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton-thick torus (N_H>10²⁴cm^–2) cannot be ruled out. We do not detect an Fe line at 6.4keV; however, the upper limit (90per cent) to the equivalent width of the 6.4keV line is high (∼3keV). Overall, the results suggest that most of the X-ray emission originates in a highly obscured Seyfert 2 nucleus.