TITLE: GCN CIRCULAR NUMBER: 39968 SUBJECT: GRB 250328A: Swift/UVOT Detection DATE: 25/03/31 10:23:48 GMT FROM: noelklin@umbc.edu

N. Klingler (NASA-GSFC / UMBC / CRESST II), M.H. Siegel (PSU), and P.A. Evans (U. Leicester), report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250328A 3254 s after the SVOM trigger sb25032803 (GCN Circ. 39910). A source consistent with the XRT position (Evans et al., GCN Circ. 39967) is detected in the initial UVOT exposures and appears to be fading.

Preliminary detections and 3-sigma upper limits for the early exposures are:

Filter T_start(s) T_stop(s) Exp(s) Mag(AB) u 3254 4993 1711 21.93 +/- 0.30 u 8758 8994 232 >21.01 u 177580 190264 970 >21.88

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.018 in the direction of the burst (Schlegel et al. 1998).

This is an official data product of the Swift/UVOT team.

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