Swift X-Ray Upper Limits on Type Ia Supernova Environments
  We have considered 53 Type Ia supernovae (SNe Ia) observed by the Swift X-Ray Telescope (XRT). None of the SNe Ia are individually detected at any time or in stacked images. Using these data and assuming that the SNe Ia are a homogeneous class of objects, we have calculated upper limits to the X- ray luminosity (0.2 - 10 keV) and mass-loss rate of L0.2-10 < 1.7\times1038 erg s-1 and M \cdot < 1.1\times10-6 M\odot yr-1 \times(vw)/(10 km s-1), respectively. The results exclude massive or evolved stars as the companion objects in SNe Ia progenitor systems, but allow the possibility of main sequence or small stars, along with double degenerate systems consisting of two white dwarfs, consistent with results obtained at other wavelengths (e.g., UV, radio) in other studies. 
Brock R. Russell and Stefan Immler
astro-ph.HE