X-ray observations of the ultraluminous infrared galaxy IRAS 19254-7245 (the Superantennae)
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 3σ upper limit of X-ray luminosity LX~8×1041ergs-1 in the 0.1-2keV band. However, we obtain a clear detection by ASCA, yielding a luminosity in the 2-10keV band of 2×1042ergs-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 NH=2×1023cm-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 (NH>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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2000
- DOI:
- 10.1046/j.1365-8711.2000.03359.x
- arXiv:
- arXiv:astro-ph/9912348
- Bibcode:
- 2000MNRAS.314..589P
- Keywords:
-
- GALAXIES: ACTIVE;
- GALAXIES: INDIVIDUAL: IRAS 19254-7245 (THE SUPERANTENNAE);
- GALAXIES: STARBURST;
- X-RAYS: GALAXIES;
- Astrophysics
- E-Print:
- 5 pages, 1 figure, 1 table, To appear in MNRAS