088462

We propose a deep observation of DEM L71 a middle-aged Type Ia SNR in the LMC primarily using the Reflection Grating Spectrometers (RGS). Previous XMM-Newton and Chandra studies of this SNR revealed the centrally-peaked emission from the Fe-rich ejecta indicating that the whole supernova ejecta have been heated by the reverse shock. Our analysis of the existing RGS data indicates an unusually low Ni/Fe mass ratio although its uncertainties are somewhat large. Theoretically such low Ni/Fe ratio is expected only for a thermonuclear explosion of a sub-Chandrasekhar-mass white dwarf (sub-MCh WD). With this proposed observation we will put a stringent constraint on the Ni/Fe ratio providing the first robust evidence of the sub-MCh progenitor for a single Type Ia SN/SNR. [truncated! Please see actual data for full text]