X-ray observations of the ultraluminous infrared galaxy IRAS 19254-7245 (the Superantennae)

We present ROSAT High Resolution Imager (HRI) and ASCA observations of the well-known ultraluminous infrared galaxy (ULIRG) IRAS 19254-7245 (the 'Supe rantennae'). The object is not detected by ROSAT, implying a 3 sigma upper limit of X-ray luminosity L(X)similar to 8x10(41) erg s(-1) in the 0.1-2 ke V band. However, we obtain a clear detection by ASCA, yielding a luminosity in the 2-10 keV band of 2x10(42) erg s(-1). The X-ray spectrum of IRAS 192 54-7245 is very hard, equivalent to a photon index of Gamma=1.0 +/- 0.35. W e therefore attempt to model the X-ray data using a 'scatterer' model, in w hich 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 sig ht 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 absorbin g column density of N-H=2x10(23) cm(-2) for a power-law photon index of Gam ma=1.9, an order of magnitude above the column estimated on the basis of op tical observations; the percentage of the scattered emission is high (simil ar to 20 per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton-thick torus (N-H> 10(24) cm(-2)) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper l imit (90 per cent) to the equivalent width of the 6.4 keV line is high (sim ilar to 3 keV). Overall, the results suggest that most of the X-ray emissio n originates in a highly obscured Seyfert 2 nucleus.