TITLE: GCN CIRCULAR NUMBER: 41471 SUBJECT: GRB 250821A: Swift/UVOT Upper Limits DATE: 25/08/21 17:55:04 GMT FROM: Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk> S. R. Oates (Lancaster U.) and S. Dichiara (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250821A 147 s after the BAT trigger (Dichiara et al., GCN Circ. 41462). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 41463) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL white (FC) 147 297 148 >20.8 u (FC) 455 475 19 >18.3 white 147 1715 281 >21.1 v 381 1593 136 >19.1 b 480 1692 127 >19.9 u 455 1667 136 >19.6 uvw1 431 1643 58 >18.7 uvm2 406 1618 136 >19.1 uvw2 357 1569 136 >19.4 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.015 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41471. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...