TITLE: GCN CIRCULAR NUMBER: 40244 SUBJECT: GRB 250424A: Swift/UVOT Detection DATE: 25/04/24 23:09:21 GMT FROM: Mike Siegel at PSU/Swift MOC mhs18@psu.edu

M.H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250424A 254 s after the BAT trigger (Cenko et al., GCN Circ. 40224). A source consistent with the XRT position (Evans et al., GCN Circ. 40232) and the optical transient (Francile et al., GCN Circ. 40222, Brivio et al., GCN Circ. 40225; Becerra et al., GCN Circ. 40226; de Wet et al., GCN Circ. 40229; Ducoin et al., GCN Circ. 40230; Turpin et al., GCN Circ. 40240; Dutton et al., GCN Circ. 40241) is detected in the initial UVOT exposures.

The preliminary UVOT position is: RA (J2000) = 14:29:60.00 = 217.49998 (deg.) Dec (J2000) = -35:01:30.6 = -35.02518 (deg.) with an estimated uncertainty of 0.47 arc sec. (radius, 90% confidence).

Preliminary detections 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 254 404 147 18.96 +/- 0.07 v 411 1918 175 >18.7 b 509 1844 136 19.97 +/- 0.26 u 485 1647 136 19.15 +/- 0.20 w1 460 1795 156 19.44 +/- 0.31 m2 436 1598 97 >18.4 w2 559 1894 156 >19.9

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.064 in the direction of the burst (Schlegel et al. 1998).

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