TITLE: GCN CIRCULAR NUMBER: 10661 SUBJECT: GRB 100418A: Possible Evidence for a Supernova DATE: 10/04/23 19:18:34 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/USRA/GSFC), F. E. Marshall (NASA/GSFC), M. Page (MSSL-UCL), M. de Pasquale (MSSL-UCL), and M. H. Siegel (PSU) report on behalf of the Swift/UVOT team: Swift/UVOT white photometry of GRB 100418A reveals a light curve with a rise and decay that are broadly similar to the white light curve of GRB 060218/SN2006aj. The UVOT spectral energy distribution at 6400 s is consistent with a power law with a spectral index of beta = 0.2 (f = nu^-beta), which is similar to the UVOT spectral energy distribution for GRB 060218/SN2006aj at 9300 s. In addition, the X-ray light curve of GRB 100418A is broadly similar to the X-ray light curves of GRB 100316D and GRB 060218, both of which had supernova components. Therefore, we suggest that the brightening of the afterglow of GRB 100418A seen at approximately 10,000 s (Bikamev et al. 2010, GCNC 10635; Siegel et al. 2010, GCNC 10645) may be the shock breakout from a supernova associated with this burst. If this is the case, and the supernova is similar to SN1998bw, then the supernova has not yet reached maximum light. Ground based observations to determine if a supernova is present are encouraged.