ROSAT HRI DISCOVERY OF LUMINOUS EXTENDED X-RAY-EMISSION IN NGC-6240

We report the detection of luminous extended X-ray emission in NGC 624 0 on the basis of ROSAT HRI observations of this ultraluminous IR gala xy. The spatial structure and temporal behavior of the X-ray source we re analyzed. We find that greater than or similar to 70% of the soft X -ray emission is extended beyond a radius of 5 ''. Strong emission can be traced out to a radius of 20 '' and weaker emission extends out to similar to 50 ''. With a luminosity of at least L-x similar or equal to 10(42) erg/s this makes NGC 6240 one of the most luminous X-ray emi tters in extended emission known. Evidence for a nuclear compact varia ble component is indicated by a drop of 32% in the HRI count rate as c ompared to the PSPC data taken one year earlier. No short-timescale va riability is detected. The HRI data, which represent the first high-re solution study of the X-ray emission from NGC 6240, complement previou s spectral fits to ROSAT PSPC data that suggested a two-component mode l consisting of thermal emission from shocked gas immersed in a starbu rst wind plus a powerlaw source attributed to scattered light from an obscured AGN. We discuss several models to account for the extended an d compact emission. Although pushed to its limits the starburst outflo w model is tenable for the essential part of the extended emission. Fo r the AGN-type component we propose a model consisting of a near-nucle ar 'warm scatterer' that explains the apparent fading of the X-ray flu x within a year as well as the strong FeK alpha complex seen in an ASC A spectrum.