TITLE:   GCN CIRCULAR
NUMBER:  20959
SUBJECT: GRB 170330A: Swift/UVOT Detection of a Fading Afterglow
DATE:    17/03/31 15:32:32 GMT
FROM:    Sam Emery at MSSL-UCL  <samuel.emery.15@ucl.ac.uk>

S.W.K. Emery (UCL-MSSL) and P. D'Avanzo (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170330A
109 s after the BAT trigger (D'Avanzo et al., GCN Circ. 20944).
A fading source consistent with the XRT position
(Evans GCN Circ. 20945)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  18:53:19.36 = 283.33067 (deg.)
    Dec (J2000) = -13:25:51.2  = -13.43090 (deg.)
with an estimated uncertainty of 0.46 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_FC           109          258          147         18.42 +/- 0.08
u_FC               266          516          246         >19.9
white              547         1368          225         19.94 +/- 0.21
v                  596         1418           97         >18.4
b                  522         1344           78         >19.3
u                  266         1484          336         >20.0
w1                 646         1467           97         >19.0
m2                 621         1442           97         >18.9
w2                 572         1393           78         >19.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.45 in the direction of the burst
(Schlegel et al. 1998).