TITLE: GCN CIRCULAR NUMBER: 41915 SUBJECT: GRB 250920B: Swift/UVOT Detection DATE: 25/09/20 19:22:10 GMT FROM: Paul Kuin at MSSL <npkuin@gmail.com> Paul Kuin (MSSL) and Rahul Gupta (NASA GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 250920B 99 s after the BAT trigger (Gupta et al., GCN Circ. 41898). A source consistent with the XRT position (Evans et al. GCN. Circ. 41910) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 12:29:16.07 = 187.31697 (deg.) Dec (J2000) = +47:50:14.4 = 47.83732 (deg.) with an estimated uncertainty of 0.42 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 99 249 147 13.89 +/- 0.02 v 4433 4633 197 16.53 +/- 0.05 b 3818 4018 197 16.62 +/- 0.04 u 311 452 139 13.26 +/- 0.02 w1 4843 5043 197 17.02 +/- 0.08 m2 4638 4838 197 >19.2 w2 4228 4428 197 19.21 +/- 0.25 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.011 in the direction of the burst (Schlegel et al. 1998). The observation also included a 49s long UV grism exposure. Inspection of the noisy grism spectrum suggests a redshift of 2.2 based on identifying an broad absorption with the the Ly-alpha line. View this GCN Circular online at https://gcn.nasa.gov/circulars/41915. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...