TITLE: GCN CIRCULAR NUMBER: 6220 SUBJECT: GRB070318: Swift UVOT indicates unusual optical/UV afterglow DATE: 07/03/21 21:08:42 GMT FROM: Mat Page at MSSL/Swift M.J. Page (MSSL-UCL), F.E. Marshall (NASA/GSFC) S. Immler (GSFC/USRA) S. T. Holland (CRESST/GSFC/USRA) and J.R. Cummings (GSFC/UMBC) report on behalf of the Swift/UVOT team: The optical/UV afterglow of GRB 070318 (Cummings et al, GCN 6210) decayed to magnitude of 21.5 in the UVOT white filter in the first 24 hours after the BAT trigger, and has remained at approximately constant brightness for at least 48 hours since that time. Examination of the Supercosmos scan of the UK Schmidt sky survey plate at this location reveals no evidence for a host galaxy (the plate limit is Bj>22 mag). For the flattening of the lightcurve to be due to the host galaxy, the host galaxy would have to be very luminous (Mb ~ -22) and have an unusually blue spectrum (e.g. peaking in the U band, observer frame). It therefore seems more likely that this is an afterglow with a very unusual lightcurve. Swift continues to monitor the afterglow. Further ground-based observations are encouraged.