Reducing the gravitational lensing scatter of type Ia supernovae without introducing any extra bias

¹ University of Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK e-mail: jacke@astro.ox.ac.uk
² Physics Department, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
³ Stockholm Observatory, AlbaNova, Department of Astronomy, 10691 Stockholm, Sweden
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark

Received: 25 September 2008
Accepted: 14 October 2008

Abstract

Aims. Magnification and de-magnification due to gravitational lensing will contribute to the brightness scatter of type Ia supernovae (SNe Ia). The purpose of this paper is to investigate the possibility of decreasing this scatter by correcting individual SNe Ia using observations of galaxies in the foreground, without introducing any extra bias.

Methods. We simulate a large number of SN Ia lines of sight populated by galaxies. For each line of sight the true magnification factor and an estimate thereof are calculated. The estimated magnification factor corresponds to what an observer would infer from a survey like the SNLS. Using the simulated data we investigate the possibility of estimating the magnification of individual supernovae with enough precision to be able to correct their brightness for gravitational lensing with negligible bias.

Results. Our simulations show that the bias arising from gravitational lensing corrections of individual SNe Ia is negligible for current and next generation surveys and that the scatter from lensing can be reduced by approximately a factor of 2. The total scatter in the SN Ia magnitudes could be reduced by 4% for an intrinsic dispersion of 0.13 mag. For an intrinsic dispersion of 0.09 mag, which may be feasible for future surveys, the total scatter could be reduced by 6%. This will reduce the errors on cosmological parameters derived from supernova data by 4–8%. The prospect of correcting for lensing is thus very good.