Discovery and progenitor constraints on the Type Ia supernova 2013gy

\nWe present an early-phase g-band light curve and visual-wavelength spectra of the normal Type Ia supernova (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., &lt; 48 h) 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 56648.5 ± 0.1). The estimate of the time of first light is consistent with the explosion time inferred from the evolution of the Si <sc>II</sc>λ6355 Doppler velocity. Furthermore, discovery photometry and previous nondetection limits enable us to constrain the companion radius down to Rc ≤ 4 R⊙. 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 &lt; 0.018 M⊙. Combined, these limits effectively rule out H-rich nondegenerate companions.\n