X-ray observations of the ultraluminous infrared galaxy IRAS 19254−7245 (the Superantennae) |
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| Authors: | A Pappa I Georgantopoulos G C Stewart |
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| Institution: | Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH; Astronomical Institute, National Observatory of Athens, Lofos Koufou, Palaia Penteli, 15236, Athens, Greece |
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| Abstract: | 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 3 σ upper limit of X-ray luminosity L X～8×1041 erg s?1 in the 0.1–2 keV band. However, we obtain a clear detection by ASCA , yielding a luminosity in the 2–10 keV band of 2×1042 erg s?1. 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=2×1023 cm?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 (～20 per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton-thick torus ( N H>1024 cm?2) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper limit (90 per cent) to the equivalent width of the 6.4 keV line is high (～3 keV). Overall , the results suggest that most of the X-ray emission originates in a highly obscured Seyfert 2 nucleus. |
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| Keywords: | galaxies: active galaxies: individual: IRAS 19254?7245 (the Superantennae) galaxies: starburst X-rays: galaxies |
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