TITLE: GCN CIRCULAR NUMBER: 40471 SUBJECT: GRB 250516A: Swift/UVOT Detection DATE: 25/05/16 22:17:55 GMT FROM: Sam Shilling at Lancaster University shilling.sam@gmail.com

S. P. R. Shilling (Lancaster U.) and A. P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250516A 108 s after the BAT trigger (Beardmore et al., GCN Circ. 40467). A source consistent with the enhanced XRT position (Evans et al., GCN Circ. 40468) is detected in the initial UVOT exposures.

The preliminary detection 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 108 1715 411 20.5 +/- 0.3 v 650 1765 136 >18.5 b 576 1691 117 >19.2 u 320 1838 362 >19.7 w1 699 1814 117 >18.8 w2 626 1741 97 >19.1

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.062 in the direction of the burst (Schlegel et al. 1998).

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