Type Ia supernovae are exploding stars that are used to measure the
accelerated expansion of the Universe and are responsible for most of the iron
ever produced. Although there is general agreement that the exploding star is a
white dwarf in a binary system, the exact configuration and trigger of the
explosion is unclear, which could hamper their use for precision cosmology. Two
families of progenitor models have been proposed. In the first, a white dwarf
accretes material from a companion until it exceeds the Chandrasekhar mass,
collapses and explodes. Alternatively, two white dwarfs merge, again causing
catastrophic collapse and an explosion. It has hitherto been impossible to
determine if either model is correct. Here we report the discovery of an object
in pre-supernova archival X-ray images at the position of the recent type Ia
supernova (2007on) in the elliptical galaxy NGC 1404. Deep optical images (also
archival) show no sign of this object. From this we conclude that the X-ray
source is the progenitor of the supernova, which favours the accretion model
for this supernova, although the host galaxy is older (6-9 Gyr) than the age at
which the explosions are predicted in the accreting models.Comment: Published in Nature See also the two follow-up papers: Roelofs,
Bassa, Voss, Nelemans Nelemans, Voss, Roelofs, Bassa both on astro-ph
02/15/0
