Evidence for Weak Lensing of Supernovae

Due to the deflection of light by density fluctuations along the line of sight, weak lensing is an unavoidable systematic uncertainty in the use of type Ia supernovae (SNe Ia) as cosmological distance indicators. We derive the expected weak lensing signatures of SNe Ia by convolving the intrinsic distribution in SN Ia peak luminosity with magnification distributions of point sources. We find that current SN Ia data at high z show both the presence of a non-Gaussian tail at the bright end (possibly due to high magnifications), and a faint-ward shift of the peak of the distribution (possibly due to demagnifications). Weak lensing effect may have begun to set in as a non-negligible systematic uncertainty in the use of SNe Ia as a cosmological probe, and should be minimized by flux-averaging. Subject headings: cosmology: observations—cosmology: theory—gravitational lensing