TITLE: GCN CIRCULAR NUMBER: 30832 SUBJECT: GRB 210912A: Swift/UVOT Detection DATE: 21/09/13 07:47:54 GMT FROM: Paul Kuin at MSSL <npkuin@gmail.com> N. P. M. Kuin (UCL-MSSL) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210912A 60 s after the BAT trigger (Troja et al., GCN Circ. 30820). A source consistent with the optical position (Hu et al. GCN Circ. 30823) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 08:39:34.27 = 129.89280 (deg.) Dec (J2000) = +79:13:17.1 = 79.22141 (deg.) with an estimated uncertainty of 0.47 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 60 209 147 18.73 +/- 0.08 v 602 621 19 >17.8 b 528 547 19 >18.3 u 272 490 214 19.56 +/- 0.31 w2 577 597 19 >18.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.027 in the direction of the burst (Schlegel et al. 1998).