TITLE: GCN CIRCULAR NUMBER: 18224 SUBJECT: GRB 150821A: Flattening in the UVOT light curve DATE: 15/09/01 15:33:59 GMT FROM: Paul Kuin at MSSL Paul Kuin, Massimiliano de Pasquale (UCL/MSSL), Daniele Malesani (DARK/NBI) and Peter Brown (TAMU) report on behalf of the Swift UVOT team: The light curve from GRB 150821A as observed by Swift/UVOT has been monitored continuously since its discovery, up to about 300 ks in all seven filters, and in u, v and white since. The GRB brightness decreased as a straight power law with index -1.32 (+0.06/-0.05) until about 10 ks. At 100 ks there was a clear flattening in the light curve, and around 250 ks a slight brightening may have taken place, but the brightness could also be flat within our observational errors. Data taken at 835 ks are consistent with a flat or slightly decreasing brightness. The summed 5 ks image in the white band from 300 - 800 ks shows a nearby extended object to the north-west with white = 22.01 +/- 0.17 mag, slightly brighter than the GRB at about 10" distance which currently is at white=23.2 +/- 0.7 magnitudes, and is located about 5" from the edge of the nearby object. At this time it is unclear whether the observed flattening is due to the host galaxy (hosts at this distance have typically a size of 1" only) or are the emergence of a supernova contribution to the afterglow. At the redshift of z = 0.755 (D'Elia et al., GCN Circ. 18187), the observed brightness would imply a SN significantly brighter than those typically associated with GRBs. Further ground observations are encouraged.