TITLE:   GCN CIRCULAR
NUMBER:  28507
SUBJECT: GRB 200925B: Swift/UVOT Detection
DATE:    20/09/26 20:14:25 GMT
FROM:    Samantha Oates at MSSL  <samantha.oates@alumni.ucl.ac.uk>

S. R. Oates (U.Birmingham) and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 200925B
82 s after the BAT trigger (Cenko et al., GCN Circ. 28499).
A source consistent with the XRT position (Beardmore et al. GCN Circ. 28503)
and the optical position reported by the OSN
(de Ugarte Postigo et al., GCN Circ. 28501) and MASTER
(Lipunov et al., GCN Circ. 28502) is detected in the initial UVOT exposures.

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)      82          232           147        19.80 +/- 0.20
white           574         1714          264        20.77 +/- 0.37
v               623         1763          136        >18.9
b               549         1689          117        >19.6
u (FC)          294         544           246        >19.7
u               294         1663          343        >19.9
w1              672         1813          136        >19.0
m2              1249        1441           39        >17.7
w2              599         1739          117        >18.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).