SN Ia supernovae observed by Swift/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 L_0.2-10_<1.7x10^38^erg/s and \dot{M}<1.1x10^-6^M_{sun}_/yr x (v_w_)/(10km/s), respectively.

The results exclude massive or evolved stars as the companion objects in SN 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.