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).