On the detection of the progenitor of the type Ia supernova 2007on
  We present new Chandra X-ray observations and detailed astrometry of the field of the type Ia supernova 2007on, for which the detection of a likely progenitor in archival Chandra data was recently reported.   No source is detected in the new Chandra images, taken six weeks after optical maximum. We calculate a 90-99% probability that any X-ray source near the position of the supernova (SN) is fainter than in the pre-outburst images, depending on the choice of aperture, which supports the identification of the archival X-ray source with the SN.   Detailed astrometry of the X-ray and new optical images, however, gives an offset between the supernova and the measured X-ray source position of 1.15+/-0.27". Extensive simulations show that the probability of finding an offset of this magnitude is ~1%, equal to the (trial-corrected) probability of a chance alignment with any X-ray source in the field. This casts doubt on the identification of the X-ray source with the progenitor, although the scenario in which at least some of the observed X-rays are connected to the supernova may be the least unlikely based on all available data.   After a brief review of the auxiliary evidence, we conclude that only future X-ray observations can shed further light on the proposed connection between the X-ray source and the progenitor of SN 2007on, and thus whether an accreting white dwarf scenario is truly favoured for this SN Ia. 
Gijs Roelofs (CfA), Cees Bassa (McGill), Rasmus Voss (MPE), Gijs   Nelemans (Nijmegen)