TITLE:   GCN CIRCULAR
NUMBER:  20217
SUBJECT: GRB 161129A: Swift/UVOT Detection
DATE:    16/11/29 21:20:05 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

N. P. M. Kuin (UCL-MSSL) and D. Kocevski (NASA/GSFC/ORAU) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 161129A
86 s after the BAT trigger (Kocevski et al., GCN Circ. 20210).
A source consistent with the XRT position (Beardmore et al. GCN Circ.
20211) is detected in the initial UVOT exposures.

The source is detected in UVOT images at position:
RA =  21h 04m 54.6s (J2000)
Dec = 32d 08' 05.52" (J2000)
with an uncertainty of 0.6" (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               86         6356          400         20.61 +/- 0.20
v                 3858         5493          393        >19.0
b                  554         6314          413        >20.0
u                  298         6109          639        >19.8
w1                4268         5903          393        >19.4
m2                4063         5698          393        >19.6
w2                 604         5288          208        >19.5

There are low-significance signs of the source in all other filters. Though
the source seems to reach peak brightness around 500s the photometric
errors are also consistent with a constant source brightness up to 10ks.

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.21 in the direction of the burst
(Schlegel et al. 1998).