Limits on the X-ray and optical luminosity of the progenitor of the type   Ia supernova SN2007sr
  We present HST/WFPC2, GALEX and Chandra observations of the position of the type Ia supernova SN2007sr in the Antennae galaxies, taken before the explosion. No source is found in any of the observations, allowing us to put interesting constraints on the progenitor luminosity. In total there is about 450 kilosecond of Chandra data, spread over 7 different observations. Limiting magnitudes of FUV (23.7 AB mag), NUV (23.8 AB mag), F555W (26.5 Vega mag) and F814W (24.5-25 Vega mag) are derived. The distance to the Antennae is surprisingly poorly known, with almost a factor of 2 difference between the latest distance based on the tip of the red giant branch (13.3 Mpc) and the distance derived from the SN2007sr light curve (25 Mpc). Using these distances we derive limits on absolute optical and UV magnitudes of any progenitor but these are still above the brightest (symbiotic) proposed progenitors. From the Chandra data a 3 sigma upper limit to the X-ray luminosity of 0.5 -- 8.0 10^37 erg/s in the 0.3-1 keV range is found. This is below the X-ray luminosity of the potential progenitor of the type Ia supernova 2007on that we recently discovered and for which we report a corrected X-ray luminosity. If that progenitor is confirmed it suggests the two supernovae have different progenitors. The X-ray limit is comparable to the brightest supersoft X-ray sources in the Galaxy, the LMC and the SMC and significantly below the luminosities of the brightest supersoft and quasi-soft X-ray sources found in nearby galaxies, ruling out such sources as progenitors of this type Ia supernova. 
Gijs Nelemans (Nijmegen), Rasmus Voss (MPE), Gijs Roelofs (CfA), Cees   Bassa (McGill)