TITLE:   GCN CIRCULAR
NUMBER:  28774
SUBJECT: GRB 201024A: Swift/UVOT Detection
DATE:    20/10/25 00:02:46 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

N. P. M. Kuin (UCL-MSSL) and F. E. Marshall (NASA/GSFC) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 201024A
495 s after the BAT trigger (Marshall et al., GCN Circ. 28761).  A source
consistent with the XRT position (Evans et al. GCN Circ. 28770) 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              495          645          148         17.94 +/- 0.05
v                  653          845           39         17.39 +/- 0.30
b                  750         1348           58         18.65 +/- 0.32
u                  726         1151           39         17.86 +/- 0.29
w1                 702         1471           78         17.46 +/- 0.30
m2                 677         2140          117        >18.4
w2                 801         2091          117        >17.7

The observations show a rapid decay as reported by MASTER (Lipunov et al,
GCN Circ. No. 28762).
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.03 in the direction of the burst
(Schlegel et al. 1998).