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tokens: Observatory Ultraviolet Optical Telescope. A large uncertainty in the bolometric luminosity in the first epoch is due to the fact that ultraviolet images were obtained four hours after the first optical and infrared ones—a delay comparable to the predicted evolution timescale of the emission at that time. Therefore the bolometric luminosity at the epoch is calculated in two different ways: 1. By fitting a blackbody only to the simultaneous optical and infrared data (blue fit in Figure 2 ). 2. By fitting a blackbody to the optical, infrared, and (four-hour later) ultraviolet data (red fit in Figure 2 ). Figure 2. Blackbody fits to the optical and infrared SED at discovery (black points; blue fit) and to the ultraviolet, optical, and infrared (the ultraviolet data are from four hours after discovery; gray points; red fit). Including the ultraviolet data reduces the inferred blackbody temperature by ∼1500 K, resulting in a lower bolometric luminosity at discovery (Figure 5 ). Optical and infrared magnitudes interpolated to the ultraviolet epoch are shown in purple empty circles for comparison and demonstrate the rapid evolution of the SED at these times. The insets show the corner plots for each MCMC fit, with contour lines denoting 50% and 90% bounds, red and blue solid lines (overplotted on each histogram) denoting the mean and median of each parameter distribution (on top of each other in most cases), and dashed lines denoting 68% confidence bounds. The bandpasses of the different filters used are shown at the bottom of the main plot. Both fits for the discovery epoch are presented in Figure 2 . They each match the data well and converge cleanly on a set of blackbody parameters. Including the ultraviolet data from four hours after discovery in the discovery epoch lowers the best-fit temperature by about 1500 K, compared to not having any ultraviolet constraints. A total of 21 additional epochs are fit. All fits converge onto single-peaked radius and temperature distributions except in five epochs where a small number of walkers converged on disjointed local likelihood maxima, leading to a >15% relative error in the radius, temperature, or deduced bolometric luminosity. These epochs were removed. The resulting bolometric light curve is presented and compared to other published light curves in Figure 3 . Figure 3. Bolometric light curve of the GW170817 kilonova derived here compared to those from Cowperthwaite et al. ( 2017 ), Drout et al. ( 2017 ), and Waxman et al. ( 2017 , SED integration method). After the first day, the bolometric light curve declines at an initial rate of ∼ t −1 followed by a steeper ∼ t −1.3 decline at later times, consistent with the expected radioactive heating rate. It is possible that the true SED of the kilonova cannot be described by a single blackbody in all epochs. However, the good fits in most epochs suggest that the blackbody approximation is reasonable. In addition, the bolometric values derived here are