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 lightcurves of
spectroscopic and photometrically identified SNe Ia to fit the rising part of
the lightcurve with a simple quadratic model. We obtain a lightcurve 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. 2006) 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 lightcurve 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
solar masses of radioactive nickel synthesized in the explosion. Our
conclusions are the same for the single-stretch and two-stretch
parameterizations of the lightcurve.
