Early light curve of SN 2013gy

The light curve is constructed by determining the appropriate S-corrections to transform KAIT natural-system B- and V-band photometry and Carnegie Supernova Project natural-system g-band photometry to the Pan-STARRS1 g-band natural photometric system. A Markov chain Monte Carlo calculation provides a best-fit single power-law function to the first ten epochs of photometry described by an exponent of 2.16^+0.06^_0.06_ and a time of first light of MJD 56629.4^+0.1^_0.1_, which is 1.93^+0.12^_0.13_ days (i.e., <48hr) before the discovery date (2013 December 4.84 UT) and -19.10^+0.12^_0.13_ days before the time of B-band maximum (MJD 56648.50.1).

The estimate of the time of first light is consistent with the explosion time inferred from the evolution of the SiII 6355 Doppler velocity. Furthermore, discovery photometry and previous nondetection limits enable us to constrain the companion radius down to Rc<=4R_{sun}_. In addition to our early-time constraints, we used a deep +235 day nebular-phase spectrum from Magellan/IMACS to place a stripped H-mass limit of <0.018M_{sun}_.