TITLE:   GCN CIRCULAR
NUMBER:  29811
SUBJECT: GRB 210411C: Swift/UVOT Detection
DATE:    21/04/12 22:05:23 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

N. P. M. Kuin (UCL-MSSL) and Sbarrato (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210411C
66 s after the BAT trigger (Sbarrato et al., GCN Circ. 29794).
A source consistent with the XRT position (Goad et al. GCN Circ. 29800)
is detected in the initial UVOT exposures. Further detections were
reported by Strausbaugh et al. (GCN Circ 29802)

The preliminary UVOT position is:
    RA  (J2000) =  19:46:26.81 = 296.61170 (deg.)
    Dec (J2000) = -39:23:52.2  = -39.39784 (deg.)
with an estimated uncertainty of 0.43 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               66          216          147         17.16 +/- 0.03
v                  609          801           39         17.43 +/- 0.18
b                  535          555           20         17.82 +/- 0.17
u                  279          528          246         17.82 +/- 0.07
w1                 658         1280           78        >19.7
m2                 633         5836           83        >19.6
w2                 584         5600          255        >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.081 in the direction of the burst
(Schlegel et al. 1998).