1. Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85748, Garching, Germany
2. Excellence Cluster Universe, Boltzmannstrasse 2, 85748, Garching, Germany
3. Department of Astrophysics, IMAPP, Radboud University, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands

Correspondence to: Rasmus Voss^1,2Gijs Nelemans³ Correspondence and requests for materials should be addressed to R.V. (Email: rvoss@mpe.mpg.de) or G.N. (Email: nelemans@astro.ru.nl).

Abstract

Type Ia supernovae are exploding stars that are used to measure the accelerated expansion of the Universe^{1, 2} and are responsible for most of the iron ever produced³. 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 unclear⁴, 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^{5, 6}. Alternatively, two white dwarfs merge, again causing catastrophic collapse and an explosion^{7, 8}. 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.

1. Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85748, Garching, Germany
2. Excellence Cluster Universe, Boltzmannstrasse 2, 85748, Garching, Germany
3. Department of Astrophysics, IMAPP, Radboud University, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands

Correspondence to: Rasmus Voss^1,2Gijs Nelemans³ Correspondence and requests for materials should be addressed to R.V. (Email: rvoss@mpe.mpg.de) or G.N. (Email: nelemans@astro.ru.nl).

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