Weak lensing of the high redshift SNIa sample

The current sample of high-redshift Type Ia supernovae (SNeIa), which combines results from two teams, the High-z Supernova Search Team and the Supernova Cosmology Project, is analysed for the effects of weak lensing. After correcting supernovae (SN) magnitudes for cosmological distances, assuming recently published, homogeneous distance and error estimates, we find that brighter SNe are preferentially found behind regions (5-15 arcmin radius) that are overdense in foreground, z ∼ 0.1 galaxies. This is consistent with the interpretation that SN fluxes are magnified by foreground galaxy excess and demagnified by foreground galaxy deficit, compared with a smooth universe case. The difference between most magnified and most demagnified SNe is approximately 0.3-0.4 mag. The effect is significant at the >99 per cent level. Simple modelling reveals that the slope of the relation between supernova magnitude and foreground galaxy density depends on the amount and distribution of matter along the line of sight to the sources, but does not depend on the specifics of the galaxy biasing scheme.