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The explosion of a Type Ia supernova, SN 2011ef, in the nearby Pinwheelgalaxy (M101 at 6.4 Mpc) provides an opportunity to study pre-explosion imagesand search for the progenitor, which should consist of a white dwarf (WD),possibly surrounded by an accretion disk, in orbit with another star. We reporton our use of deep Chandra observations to limit the luminosity and temperatureof the pre-explosion white dwarf (WD). It is found that if the spectrum was ablackbody, then WDs of highest possible temperatures and luminosities areexcluded but, even if the WD was emitting at the Eddington luminosity, valuesof kT less than roughly 60 eV are permitted. This allows the progenitor to bean accreting nuclear-burning WD with an expanded photosphere. Pre-SN HSTobservations were used to derive a lower limit of about 10 eV for the expandedphotosphere. Li et al.\, (2011) have already ruled out the possibility of agiant donor. We consider the combined emission from the WD, disk, and donor,and find that even the combined emission from a bright subgiant, WD and diskwould not likely have been observed prior to explosion, and neither would somelocal candidates for the nuclear-burning WD model.

On the Nature of the Progenitor of the Type Ia SN2011fe in M101