TITLE:   GCN CIRCULAR
NUMBER:  41975
SUBJECT: GRB 250924A: Swift/UVOT Detection
DATE:    25/09/24 23:25:48 GMT
FROM:    Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>

M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250924A
193 s after the BAT trigger (Siegel et al., GCN Circ. 41959).
A source consistent with the XRT position is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  03:27:29.66 =  51.87358 (deg.)
    Dec (J2000) = +74:39:34.0  =  74.65945 (deg.)
with an estimated uncertainty of 0.43 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              193          342          147         17.80 +/- 0.04
v                  681         1234           58        >18.4
b                  606          626           19         18.75 +/- 0.30
u                  351          601          246         18.03 +/- 0.08
w1                 730         1110           39        >19.0
m2                1065         1085           19        >17.6
w2                 656         1210           78        >19.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.315 in the direction of the burst
(Schlegel et al. 1998).