Left: estimate of t_explosion based upon functional fits to the velocity evolution of Si IIλ6355. Right: maximum allowed radius of a companion star as a function of explosion time as inferred from the discovery g_PS1 photometry and the previous KAIT nondetection limit plotted vs. days since discovery. The vertical black line in the left and right panels corresponds to the best-fit t_first, accompanied by a shaded gray region corresponding to its 1σ uncertainty. Bottom: rest-frame-corrected nebular spectrum of SN 2013gy (black line), the continuum fit (red), and our 3σ Hα flux limit (blue line). The shaded area extends over the expected location of Hα ± 1000 km s⁻¹.