TITLE: GCN CIRCULAR NUMBER: 30975 SUBJECT: GRB 211023B: Swift/UVOT Detection DATE: 21/10/24 18:21:49 GMT FROM: Paul Kuin at MSSL <npkuin@gmail.com> N. P. M. Kuin (UCL-MSSL) 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 211023B 98 s after the BAT trigger (Dichiara et al., GCN Circ. 30960). A source consistent with the XRT position (Beardmore et al. GCN Circ. 30966) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 11:21:14.33 = 170.30971 (deg.) Dec (J2000) = +39:08:08.8 = 39.13579 (deg.) with an estimated uncertainty of 0.53 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 98 247 147 19.44 +/- 0.11 v 640 1753 136 >18.8 b 566 1679 117 >19.9 u 310 734 265 19.50 +/- 0.20 w1 690 1803 117 >19.3 w2 615 1039 39 >19.9 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.024 in the direction of the burst (Schlegel et al. 1998).