The Rise Time of Normal and Subluminous Type Ia Supernovae

We calculate the average stretch-corrected rise time of Type Ia supernovae (SNe Ia) in the Supernova Legacy Survey. We use the aggregate light curves of spectroscopic and photometrically identified SNe Ia 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 et al.) 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 SNe Ia 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 SNe Ia (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 SNe Ia 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.

Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products at the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.