X-ray emission from a merger remnant, NGC 7252,the "Atoms-for-Peace" galaxy

Abstract

We observed a nearby merger remnant NGC 7252 with the X-ray satellite ASCA, and detected X-ray emission with the X-ray flux of (1.8+/-0.3)E-13 ergs/s/cm/cm in the 0.5 - 10 keV band. This corresponds to the X-ray luminosity of 8.1E40 ergs/s. The X-ray emission is well described with a two-component model: a soft component with kT =0.72+/-0.13 keV and a hard component with kT > 5.1 keV. Although NGC 7252 is referred to as a dynamically young protoelliptical, the 0.5 - 4 keV luminosity of the soft component is about 2E40 ergs/s, which is low for an early-type galaxy. The ratio of L_X/L_FIR suggests that the soft component originated from the hot gas due to star formation. Its low luminosity can be explained by the gas ejection from the galaxy as galaxy winds. Our observation reveals the existence of hard X-ray emission with the 2 - 10 keV luminosity of 5.6E40 ergs/s. This may indicate the existence of nuclear activity or intermediate-mass black hole in NGC 7252.