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

We present a set of 11 type Ia supernova (SN Ia) lightcurves with dense,pre-maximum sampling. These supernovae (SNe), in galaxies behind the LargeMagellanic Cloud (LMC), were discovered by the SuperMACHO survey. The SNe spana redshift range of z = 0.11 - 0.35. Our lightcurves contain some of theearliest pre-maximum observations of SNe Ia to date. We also give a functionalmodel that describes the SN Ia lightcurve shape (in our VR-band). Our functionuses the "expanding fireball" model of Goldhaber et al. (1998) to describe therising lightcurve immediately after explosion but constrains it to smoothlyjoin the remainder of the lightcurve. We fit this model to a composite observedVR-band lightcurve of three SNe between redshifts of 0.135 to 0.165. These SNehave not been K-corrected or adjusted to account for reddening. In thisredshift range, the observed VR-band most closely matches the rest frameV-band. Using the best fit to our functional description of the lightcurve, wefind the time between explosion and observed VR-band maximum to be17.6+-1.3(stat)+-0.07(sys) rest-frame days for a SN Ia with a VR-band Deltam_{-10} of 0.52mag. For the redshifts sampled, the observed VR-bandtime-of-maximum brightness should be the same as the rest-frame V-band maximumto within 1.1 rest-frame days.