The rise time of normal and subluminous type Ia supernovae

Abstract

We calculate the average stretch-corrected rise time of Type Ia supernovae (SNeIa) in the Supernova Legacy Survey. We use the aggregate light curves of spectroscopic and photometrically identified SNeIa to fit the rising part of the light curve with a simple quadratic model. We obtain a light curve shape corrected, i.e., stretch-corrected, fiducial rise time of 17.02 +0.18-0.28 (stat) days. The measured rise time differs from an earlier finding by the SNLS (Conley etal.) due to the use of different SN Ia templates. We compare it to nearby samples using the same methods and find no evolution in the early part of the light curve of SNeIa up to z = 1. We search for variations among different populations, particularly subluminous objects, by dividing the sample in stretch. Bright and slow decliners (s > 1.0) have consistent stretch-corrected rise times compared to fainter and faster decliners (0.8 < s ≤ 1.0); they are shorter by 0.57+0.47 -0.50 (stat) days. Subluminous SNeIa (here defined as objects with s ≤ 0.8), although less constrained, are also consistent, with a rise time of 18.03+0.81-1.37 (stat) days. We study several systematic biases and find that the use of different fiducial templates may affect the average rise time but not the intrinsic differences between populations. Based on our results, we estimate that subluminous SNeIa are powered by 0.05-0.35 M⊙ of 56Ni synthesized in the explosion. Our conclusions are the same for the single-stretch and two-stretch parameterizations of the light curve.