TITLE: GCN CIRCULAR NUMBER: 37262 SUBJECT: GRB 240821A: Swift/UVOT observations of the XRT sources. DATE: 24/08/24 16:46:55 GMT FROM: Sam Shilling at Lancaster University S. P. R. Shilling (Lancaster U.) and S. R. Oates (Lancaster U.) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 240821A 91 ks after the Fermi/GBM trigger (Fermi GBM team, GCN Circ. 37219). The UVOT exposures include the positions of XRT sources 1, 2, 3 and 4 (Bernardini et al., GCN 37249). We note there is a DSS source (J233641.30-100258.0, Rmag ~20.76) coincident with the XRT source 2 position, and a USNO-B1.0 source (0798-0668880, Bmag ~18.31) coincident with the XRT source 4 position. There are no DSS or USNO-B1.0 sources at the positions of XRT sources 1 and 3. Preliminary magnitudes, and upper limits, using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the four XRT sources are: Source Filter T_start(s) T_stop(s) Exp(s) Mag XRT source 1 u 91622 93293 1643 >20.92 XRT source 2 u 91622 93293 1643 20.73 +/- 0.30 XRT source 3 u 91622 93293 1643 >20.92 XRT source 4 u 91622 93293 1643 18.27 +/- 0.05 XRT source 1 w2 136400 153663 3261 >21.42 XRT source 2 w2 136400 153663 3261 20.55 +/- 0.18 XRT source 3 w2 136400 153663 3261 >21.43 XRT source 4 w2 136400 153663 3261 20.66 +/- 0.20 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel et al. 1998).