TITLE:   GCN CIRCULAR
NUMBER:  45735
SUBJECT: GRB 260924A: Swift/UVOT Detection
DATE:    26/09/25 01:37:57 GMT
FROM:    noelklin@umbc.edu

N. Klingler (NASA-GSFC / UMBC / CRESST II) and V. D'Elia (SSDC & INAF-OAR) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 260924A 158 s after the BAT trigger (D'Elia et al., GCN Circ. 45713).  A source consistent with the XRT position (Evans et al., GCN Circ. 45715) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  08:55:03.13 = 133.76304 (deg.)
    Dec (J2000) = +12:02:35.4  =  12.04318 (deg.)
with an estimated uncertainty of 0.50 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              158          308          147         14.68 +/- 0.02
v                 4254         4454          197         15.11 +/- 0.03
b                 3637         3837          197         15.71 +/- 0.03
u                  334          372           37         17.33 +/- 0.24
w1                4664         4864          197        >18.5
m2                4459         4658          197        >19.2
w2                4048         4247          197        >19.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.041 in the direction of the burst (Schlegel et al. 1998).