TITLE: GCN CIRCULAR NUMBER: 41800 SUBJECT: GRB 250911A: Swift/UVOT Detection DATE: 25/09/11 16:49:55 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M. H. Siegel (PSU) and N. J. Klingler (GSFC/UMBC/CRESSTII) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250911A 3.5 ks after the BAT trigger (Klingler et al., GCN Circ. 41781). A fading afterglow consistent the XRT position (Beardmore et al., GCN Circ. 41784) and COLIBRI afterglow (Angulo et al., GCN Circ. 41782) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 17:35:00.03 = 263.75011 (deg.) Dec (J2000) = -16:51:14.9 = -16.85413 (deg.) with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence). Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag/3sigUL white 3483 4252 344 19.95+/-0.16 v 4462 4662 196 >19.20 v 26629 26655 25 >17.87 b 3847 4047 196 19.40+/-0.22 b 10440 10955 503 20.58+/-0.33 u 3642 5214 330 19.18+/-0.19 u 21026 21075 48 >18.94 uvw1 4872 5072 196 >19.53 uvw1 20120 21019 885 >20.01 uvm2 4667 4867 196 >19.38 uvm2 16021 16815 782 >20.49 uvw2 4258 4458 196 >19.50 uvw2 25723 26622 885 >20.09 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.4185 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/41800. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...