Left: early-time g_PS1-band light curve of SN 2013gy based on photometry measured from PS1, CSP-II, and KAIT images, plotted along with the best-fit, single power-law function characterized by an index α = 2.16 (solid curve), the “fireball” model (α = 2, dash-dot curve), and a linear rise for the first ∼3 d (dashed curve). The KAIT open-filter nondetection obtained 1 d prior to discovery is plotted as a black filled triangle. The solid vertical black line indicates t_first with its width corresponding to the 1σ uncertainty (gray region), and the vertical dashed line corresponds to the epoch of the first spectrum. Right, top: residuals between the best-fit model and the data (horizontal dashed white line); black lines correspond to models sampled from below (right, bottom panel), which contains 1σ, 2σ, and 3σ error ellipses indicating the uncertainties in the model fit parameters α and t_first.