TITLE: GCN CIRCULAR NUMBER: 23274 SUBJECT: GRB 180925A: Swift/UVOT Detection DATE: 18/09/26 12:53:04 GMT FROM: Sam Emery at MSSL-UCL S.W.K. Emery (UCL-MSSL) and J. D. Gropp (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 180925A 119 s after the BAT trigger (Gropp et al., GCN Circ. 23268). A fading source consistent with the XRT position (Goad et al. GCN Circ. 23271) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 21:00:48.70 = 315.20292 (deg.) Dec (J2000) = -64:27:24.82 = -64.456894 (deg.) with an estimated uncertainty of 0.22 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 119 269 147 19.73 +/- 0.17 white 557 6943 674 20.32 +/- 0.13 v 607 7354 549 >19.66 b 533 6739 347 20.89 +/- 0.37 u 277 6533 579 19.78 +/- 0.21 uvw1 657 7755 540 20.07 +/- 0.29 uvm2 1062 7559 413 >20.14 uvw2 583 7149 510 >20.60 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).