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

S. P. R. Shilling (Lancaster U.), S. R. Oates (Lancaster U.) and S. Dichiara (PSU) 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 17:22:59 UT on 24/04/03, ~2.1 hours after the initial detections.

An optical afterglow is initially detected in the U-band consistent with the position of the uncatalogued X-ray source detected by Swift/XRT (Dichiara et al., GCN Circ. 40037) and 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). At 00:02:02 UT on 25/04/04, ~6.7 hours later, the optical afterglow is no longer detected.

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

Filter         T_start(h) 	   Exp(s)         Mag

u	       2.1		   396	          19.2 +/- 0.2
u              8.8		   492            >20.3
b              8.8		   386            >20.5
v              9.0		   130            >19.0
w1             12.1		   164            >19.3
m2             12.0		   245            >19.7
w2             8.9		   246            >19.8

The magnitudes given are calculated using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) and are not corrected for the Galactic extinction.