TITLE: GCN CIRCULAR NUMBER: 29697 SUBJECT: GRB 210321A: Swift/UVOT Detection DATE: 21/03/23 14:17:59 GMT FROM: Sam LaPorte at PSU GRB 210321A: Swift/UVOT Detection S. J. Laporte (PSU) and S. Dichiara (NASA/GSFC/UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210321A 2735 s after the BAT trigger (Dichiara et al., GCN Circ. 29677). A source consistent with the XRT position (Beardmore et al. GCN Circ. 29681) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 05:51:34.75 = 87.89479 (deg.) Dec (J2000) = +70:07:49.8 = 70.13051 (deg.) with an estimated uncertainty of 0.49 arc sec. (radius, 90% confidence). 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 2735 2885 147 18.69 +/- 0.09 v 2892 4465 331 18.44 +/- 0.16 b 3713 3913 197 18.64 +/- 0.11 u 3507 3706 197 18.29 +/- 0.12 w1 3302 3502 197 18.48 +/- 0.17 m2 3097 3296 197 >18.9 w2 4124 4324 197 >19.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.118 in the direction of the burst (Schlegel et al. 1998).