TITLE:   GCN CIRCULAR
NUMBER:  41920
SUBJECT: GRB 250920C: Swift/UVOT Detection
DATE:    25/09/20 21:48:29 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

N. P. M. Kuin (UCL-MSSL) and R. Gupta (GSFC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250920C 124 s
after the BAT trigger (Gupta et al., GCN Circ. 41904).  A source consistent
with the XRT position is detected in the initial UVOT exposures.

The preliminary UVOT position is:

 RA  (J2000)= 03h 25m 51.76s, 51.46568 deg
 Dec (J2000) = -29d 39m 20.2s,  -29.65562 deg

consistent with the detection by GOTO (Wortley et al. GCN Circ. 41907).

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding
chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           146          207         60.5          14.55 +/- 0.04
v                  124          133         10.1          14.56 +/- 0.09

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.019 in the direction of the burst
(Schlegel et al. 1998).