LIGO/VIRGO G298048: Expansion Velocity of SSS17a Consistent With Relativistic Merger Ejecta

We construct a preliminary UV/Optical/NIR SED for the potential optical counterpart SSS17a first reported by Coulter et al. (GCN 21529). These data are compiled from the photometry reported in circulars submitted during the first night of follow-up. We correct these data for extinction due to reddening along the line of sight to the host galaxy NGC 4993. We perform a preliminary fit with a blackbody function to this SED finding an effective temperature of T = 8000 K and an effective radius of R = 6e14 cm. We test the sensitivity of our fit to the UV flux, refitting with the Swift/UVOT data excluded. We find T = 10,200 K and R ~ 4e14 cm. In both cases, the large expansion radius and short time since explosion (t ~ 1 day) suggest an expansion velocity of >0.2c. This is too fast to be explained by supernova ejecta, but is consistent with the expansion velocities expected for relativistic BNS/NS-BH merger ejecta. These values, and the inferred relativistic expansion, are consistent with the results of Malesani et al. (GCN 21577).