Weak-lensing magnification of Type Ia supernovae from the Pantheon sample

Using data from the Pantheon Type Ia supernovae (SN Ia) compilation and the Sloan Digital Sky Survey, we propose an estimator for weak-lensing convergence incorporating positional and photometric data of foreground galaxies. The correlation between this and the Hubble diagram residuals of the supernovae has 3.6σ significance, and is consistent with weak-lensing magnification due to dark matter haloes centred on galaxies. We additionally constrain the properties of the galactic haloes, such as the mass-to-light ratio Γ and radial profile of the halo matter density ρ(r). We derive a new relationship for the additional rms scatter in magnitudes caused by lensing, finding σlens = (0.06 ± 0.017)(dC(z)/dC(z = 1))3/2, where dC(z) is the comoving distance to redshift z. Hence, the scatter in apparent magnitudes due lensing will be of the same size as the intrinsic scatter of SN Ia by z ∼1.2. We propose a modification of the distance modulus estimator for SN Ia to incorporate lensing, which can be easily calculated from observational data. We anticipate this will improve the accuracy of cosmological parameter estimation for high-redshift SN Ia data.