Light Curves of Type Ia Supernovae from Near the Time of Explosion

We present a set of 11 Type Ia supernova (SN Ia) light curves with dense, premaximum sampling. These supernovae (SNe), in galaxies behind the Large Magellanic Cloud (LMC), were discovered by the SuperMACHO survey. The SNe span a redshift range of z = 0.11-0.35. Our light curves contain some of the earliest premaximum observations of SNe Ia to date. We also give a functional model that describes the SN Ia light-curve shape (in our VR band). Our function uses the "expanding fireball" model of Goldhaber et al. to describe the rising light curve immediately after explosion but constrains it to smoothly join the remainder of the light curve. We fit this model to a composite observed VR-band light curve of three SNe between redshifts of 0.135 and 0.165. These SNe have not been K-corrected or adjusted to account for reddening. In this redshift range, the observed VR band most closely matches the rest-frame V band. Using the best fit to our functional description of the light curve, we find the time between explosion and observed VR-band maximum to be 17.6 ± 1.3(stat) ± 0.07(sys) rest-frame days for a SN Ia with a VR-band Δm-10 of 0.52 mag. For the redshifts sampled, the observed VR-band time of maximum brightness should be the same as the rest-frame V-band maximum to within 1.1 rest-frame days.