TITLE: GCN CIRCULAR NUMBER: 40048 SUBJECT: GRB 250403A: Swift/UVOT Upper Limits DATE: 25/04/04 13:28:59 GMT FROM: Sam Shilling at Lancaster University shilling.sam@gmail.com

S. P. R. Shilling (Lancaster U.) and S. R. Oates (Lancaster U.) report on behalf of the Swift UVOT team:

Swift/UVOT observed the field of GRB 250403A detected by Fermi GBM (Fermi team, GCN Circ. 40025) and SVOM/ECLAIRs (Julakanti et al., GCN Circ. 40026) starting at 00:02:02 UT on 25/04/04, ~8.75 hours after the initial detections.

No optical afterglow is detected consistent with the position of the uncatalogued X-ray source detected by Swift/XRT (Dichiara et al., GCN Circ. 40037) or the optical afterglow detected by various facilities (Starling et al., GCN Circ. 40028; Wu et al., GCN Circ. 40029; Du et al., GCN Circ. 40031; Li et al., GCN Circ. 40032; Ghosh et al., GCN Circ. 40039; Jiang et al., GCN Circ. 40041; Pérez-Fournon et al., GCN Circ. 40043; Xin et al., GCN Circ. 40045).

Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures are:

Filter Exp(s) Mag

u 492 >20.3 b 386 >20.5 v 130 >19.0 w1 164 >19.3 m2 245 >19.7 w2 246 >19.8

The magnitudes in the table are not corrected for the Galactic extinction.

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