Off-center explosions of Chandrasekhar-mass white dwarfs: an explanation of super-bright type Ia supernovae?

The recent discovery of a very bright type Ia supernova, SNLS-03D3bb ($\equiv$ SN 2003fg), in the Supernova Legacy Survey (SNLS) has raised the question of whether super-Chandrasekhar-mass white dwarf stars are needed to explain such explosions. In principle, such a progenitor could form by the mergers of two rather massive white dwarfs. Binary systems of two white dwarfs in close orbit where their total mass significantly exceeds the Chandrasekhar mass, have not yet been found. Therefore SNLS-03D3bb could establish the first clear case of a double-degenerate progenitor of a (peculiar) type Ia supernovae. Moreover, if this interpretation is correct, it casts some doubt on the universality of the calibration relations used to make SNe Ia distance indicators for cosmology. Here we present a critical discussion of the expected observational finger prints of super-Chandrasekhar-mass explosions; in important respects, these are not consistent with the observations of SNLS-03D3bb. We demonstrate that the lop-sided explosion of a Chandrasekhar-mass white dwarf could provide a better explanation.