TITLE: GCN CIRCULAR NUMBER: 29630 SUBJECT: GRB 210308A: Swift/UVOT Detection DATE: 21/03/09 01:16:01 GMT FROM: Sam LaPorte at PSU GRB 210308A: Swift/UVOT Detection J. D. Gropp (PSU) and V. D'Elia (SSDC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210308A 95 s after the BAT trigger (D'Elia et al., GCN Circ. 29619). A source consistent with the XRT position (D'Elia et al., GCN Circ. 29619) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 04:28:17.73 = 67.07389 (deg.) Dec (J2000) = +37:26:02.1 = 37.43391 (deg.) with an estimated uncertainty of 0.46 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 b 765 1189 38 >18.62 uvm2 864 1115 38 >17.52 u 338 1164 284 >19.50 v 667 1090 58 >17.84 uvw1 1120 1140 19 >17.25 uvw2 815 1231 50 >17.91 white 126 275 292 18.54+-0.07 white 618 1213 205 >20.20 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.296 in the direction of the burst (Schlegel et al. 1998).