TITLE: GCN CIRCULAR NUMBER: 40778 SUBJECT: GRB 250617B: Swift/UVOT Detection DATE: 25/06/18 13:09:34 GMT FROM: Mike Siegel at PSU/Swift MOC <mhs18@psu.edu> M. H. Siegel (PSU) and K. L. Page (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250617B 797 s after the BAT trigger (Page et al., GCN Circ. 40760). A source consistent with the XRT position (Osborne et al., GCN Circ 40767) is detected in the initial UVOT exposures. The lack of detection in the uvw2 and uvm2 filters would be consistent with the redshift reported by Corcoran et al. (GCN Circ. 40776). The preliminary UVOT position is: RA (J2000) = 22:11:35.25 = 332.89686 (deg.) Dec (J2000) = +32:43:57.1 = 32.73253 (deg.) with an estimated uncertainty of 0.44 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 797 946 147 18.18 +/- 0.05 v 1130 2528 172 18.72 +/- 0.19 b 1229 1596 58 18.91 +/- 0.18 u 1204 1397 39 18.10 +/- 0.19 w1 1179 2408 156 19.32 +/- 0.28 m2 1155 2384 117 >18.3 w2 1106 2507 175 >19.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.087 in the direction of the burst (Schlegel et al. 1998). View this GCN Circular online at https://gcn.nasa.gov/circulars/40778. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...