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Type Ia supernovae, with their remarkably homogeneous light curves andspectra, have been used as standardizable candles to measure the acceleratingexpansion of the Universe. Yet, their progenitors remain elusive. Commonexplanations invoke a degenerate star (white dwarf) which explodes uponreaching close to the Chandrasekhar limit, by either steadily accreting massfrom a companion star or violently merging with another degenerate star. Weshow that circumstellar interaction in young Galactic supernova remnants can beused to distinguish between these single and double degenerate progenitorscenarios. Here we propose a new diagnostic, the Surface Brightness Index,which can be computed from theory and compared with Chandra and VLAobservations. We use this method to demonstrate that a double degenerateprogenitor can explain the decades-long flux rise and size increase of theyoungest known Galactic SNR G1.9+0.3. We disfavor a single degenerate scenario.We attribute the observed properties to the interaction between a steep ejectaprofile and a constant density environment. We suggest using the upgraded VLAto detect circumstellar interaction in the remnants of historical Type Iasupernovae in the Local Group of galaxies. This may settle the long-standingdebate over their progenitors.

Young Remnants of Type Ia Supernovae and Their Progenitors: A Study of SNR G1.9+0.3