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).

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