Analytic photometric redshift estimator for Type Ia supernovae from the Large Synoptic Survey Telescope

Accurate and precise photometric redshifts (photo-zs) of Type Ia supernovae (SNe Ia) can enable the use of SNe Ia, measured only with photometry, to probe cosmology. This dramatically increases the science return of supernova surveys planned for the Large Synoptic Survey Telescope (LSST). In this paper we describe a significantly improved version of the simple analytic photo-z estimator proposed by Wang and further developed by Wang, Narayan & Wood-Vasey. We apply it to 55 422 simulated SNe Ia generated using the SNANA package with the LSST filters. We find that the estimated errors on the photo-zs, σ_(z_(phot))/(1+z_(phot)), can be used as filters to produce a set of photo-zs that have high precision, accuracy, and purity. Using SN Ia colours as well as SN Ia peak magnitude in the i band, we obtain a set of photo-zs with 2 per cent accuracy (with σ(z_(phot) − z_(spec))/(1 + z_(spec)) = 0.02), a bias in z_(phot) (the mean of z_(phot) − z_(spec)) of −9 × 10^(−5), and an outlier fraction (with |(z_(phot) − z_(spec))/(1 + z_(spec))| > 0.1) of 0.23 per cent, with the requirement that σ_(z_(phot))/(1+z_(phot))<0.01. Using the SN Ia colours only, we obtain a set of photo-zs with similar quality by requiring that σ_(z_(phot))/(1+z_(phot))