TITLE:   GCN CIRCULAR
NUMBER:  40321
SUBJECT: GRB 250430A: Swift/UVOT Detection
DATE:    25/05/02 12:22:02 GMT
FROM:    Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>

A.A. Breeveld (UCL-MSSL) and T. M. Parsotan (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250430A 118 s after the BAT trigger (Parsotan et al., GCN Circ. 40292).
The optical source detected in the UVOT white filter reported in that circular (and also detected by Oden & Guessoum, GCN Circ. 40295; Garnichy et al., GCN Circ. 40301; Komesh et al., GCN Circ. 40307; Zheng et al., GCN Circ. 40310 and Li et al., GCN Circ. 40311) was also detected in the UVOT U-band filter, but has since faded.

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_FC           141          291          147         19.08 ± 0.1
white              843         1856          264         19.40 ± 0.1
v                  118         1733          169        >18.9
b                  473         1831          156        >19.8
u                  448          468           20         17.91 ± 0.3
u                  621         1117           59         18.67 ± 0.3
w1                 424         1782          156        >19.0
m2                1393         1413           19        >17.3
w2                 696         1364           58        >18.6

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.092 in the direction of the burst (Schlegel et al. 1998).