Chandra Observations of the Recurrent Nova IM Normae

The recurrent nova IM Nor was observed twice in X-rays with the Chandra ACIS-S, 1 and 6 months after the optical outburst. It was not detected in the first observation, with an upper limit on the X-ray luminosity in the 0.2-10 keV range LX < 4.8 × 1030(d kpc-1)2 ergs s-1 (where d is the distance to the nova). Five months later, a hard X-ray source with LX = × 1032(d kpc)2 ergs s-1 was detected. The X-ray spectrum appears to be thermal, but we cannot rule out additional components due to unresolved emission lines. A blackbody component is likely to contribute to the observed spectrum, but it has bolometric luminosity Lbol = 2.5 × 1033(d kpc-1)2 ergs s-1; therefore, it is not sufficiently luminous to be caused by a central white dwarf that is still burning hydrogen on the surface. An optical spectrum, taken 5 months postoutburst, indicates no intrinsic reddening of the ejecta. Therefore, we conclude that the shell had already become optically thin to supersoft X-rays, but nuclear burning had turned off or was in the process of turning off at this time. We discuss why this implies that recurrent novae, even the rare ones with long optical decays like IM Nor, indicating a large envelope mass, are not statistically significant as Type Ia supernova candidates.