  There is wide agreement that Type Ia supernovae (used as standard candles for
cosmology) are associated with the thermonuclear explosions of white dwarf
stars. The nuclear runaway that leads to the explosion could start in a white
dwarf gradually accumulating matter from a companion star until it reaches the
Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in
a compact binary system. The X-ray signatures of these two possible paths are
very different. Whereas no strong electromagnetic emission is expected in the
merger scenario until shortly before the supernova, the white dwarf accreting
material from the normal star becomes a source of copious X-rays for ~1e7 yr
before the explosion. This offers a means of determining which path dominates.
Here we report that the observed X-ray flux from six nearby elliptical galaxies
and galaxy bulges is a factor of ~30-50 less than predicted in the accretion
scenario, based upon an estimate of the supernova rate from their K-band
luminosities. We conclude that no more than ~5 per cent of Type Ia supernovae
in early type galaxies can be produced by white dwarfs in accreting binary
systems, unless their progenitors are much younger than the bulk of the stellar
population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs
make a significant contribution to the supernova rate.
