TITLE: GCN CIRCULAR NUMBER: 18192 SUBJECT: GRB 150821A: Swift/UVOT Detection DATE: 15/08/21 19:28:53 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 150821A 251 s after the BAT trigger (Troja et al., GCN Circ. 18186). We use the UVOT position as reported there which is consistent with the enhanced XRT position (Goad et al. GCN Circ. 18191). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 251 401 147 16.59 +/- 0.03 v 560 580 20 17.21 +/- 0.18 b 484 504 19 17.48 +/- 0.12 u 460 479 19 16.70 +/- 0.11 w1 435 629 39 17.62 +/- 0.18 m2 410 7287 372 >19.8 w2 535 1904 156 19.85 +/- 0.28 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).