B -band light-curve rise times for eight unusually well-observed nearby Type Ia supernovæ ( SNe ) are fitted by a newly developed template-building algorithm , using light-curve functions that are smooth , flexible , and free of potential bias from externally derived templates and other prior assumptions .
From the available literature , photometric BVRI data collected over many months , including the earliest points , are reconciled , combined , and fitted to a unique time of explosion for each SN .
On average , after they are corrected for light-curve decline rate , three SNe rise in 18.81 \pm 0.36 days , while five SNe rise in 16.64 \pm 0.21 days .
If all eight SNe are sampled from a single parent population ( a hypothesis not favored by statistical tests ) , the rms intrinsic scatter of the decline-rate-corrected SN rise time is 0.96 ^ { +0.52 } _ { -0.25 } days – a first measurement of this dispersion .
The corresponding global mean rise time is 17.44 \pm 0.39 days , where the uncertainty is dominated by intrinsic variance .
This value is \approx 2 days shorter than two published averages that nominally are twice as precise , though also based on small samples .
When comparing high- z to low- z SN luminosities for determining cosmological parameters , bias can be introduced by use of a light-curve template with an unrealistic rise time .
If the period over which light curves are sampled depends on z in a manner typical of current search and measurement strategies , a two-day discrepancy in template rise time can bias the luminosity comparison by { \approx } 0.03 magnitudes .