The Rise-Time of Normal and Subluminous Type Ia Supernovae

TL;DR

This study measures the average rise-time of Type Ia supernovae using a large survey, finds no evolution with redshift, and compares different subpopulations to understand their explosion characteristics.

Contribution

It provides a new, precise measurement of the stretch-corrected rise-time of SNe Ia and compares different populations, including subluminous supernovae, using consistent methods.

Findings

Average stretch-corrected rise-time is 17.02 days.

No evolution in early lightcurve up to redshift 1.

Subluminous SNe Ia have a rise-time around 18 days.

Abstract

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.