TITLE: GCN CIRCULAR NUMBER: 41294 SUBJECT: GRB 250807B: Swift/UVOT Detection DATE: 25/08/08 16:47:22 GMT FROM: s.shilling@lancaster.ac.uk

S. P. R. Shilling (Lancaster U.) 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 250807B 154 s after the BAT trigger (Klingler et al., GCN Circ. 41268). A source consistent with the enhanced XRT position (Goad et al., GCN Circ. 41272) is detected in the initial UVOT exposures.

Preliminary detections 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 154 303 147 16.30 +/- 0.03 u_FC 313 339 26 15.05 +/- 0.06 u 50186 50647 450 19.86 +/- 0.28 v_FC 134 145 10 >17.31 v 61831 61845 14 >17.45 w1 38723 39497 761 19.64 +/- 0.20 w2 60924 61824 886 >20.8

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.016 in the direction of the burst (Schlegel et al. 1998).

View this GCN Circular online at https://gcn.nasa.gov/circulars/41294. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...