While it is clear that Type Ia supernovae ( SNe ) are the result of thermonuclear explosions in C/O white dwarfs ( WDs ) , a great deal remains uncertain about the binary companion that facilitates the explosive disruption of the WD .
Here , we present a comprehensive analysis of a unique , and large , data set of 127 SNe Ia with exquisite coverage by the Zwicky Transient Facility ( ZTF ) .
High-cadence ( 6 observations per night ) ZTF observations allow us to measure the SN rise time and examine its initial evolution .
We develop a Bayesian framework to model the early rise as a power-law in time , which enables the inclusion of priors in our model .
For a volume-limited subset of normal SNe Ia , we find the mean power-law index is consistent with 2 in the r _ { \mathrm { ZTF } } -band ( \alpha _ { r } = 2.01 \pm 0.02 ) , as expected in the expanding fireball model .
There are , however , individual SNe that are clearly inconsistent with \alpha _ { r } = 2 .
We estimate a mean rise time of 18.5 d ( with a range extending from \sim 15–22 d ) , though this is subject to the adopted prior .
We identify an important , previously unknown , bias whereby the rise times for higher redshift SNe within a flux-limited survey are systematically underestimated .
This effect can be partially alleviated if the power-law index is fixed to \alpha = 2 , in which case we estimate a mean rise time of 21.0 d ( with a range from \sim 18–23 d ) .
The sample includes a handful or rare and peculiar SNe Ia .
Finally , we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to Large Synoptic Survey Telescope observations .