TITLE:   GCN CIRCULAR
NUMBER:  20894
SUBJECT: GRB 170317A: Swift/UVOT Detection
DATE:    17/03/17 17:00:57 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

N. P. M. Kuin (UCL-MSSL) and S. J. LaPorte (PSU) report on behalf of
the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB
170317A 87 s after the BAT trigger (LaPorte et al., GCN Circ.20885).
A fading source consistent with the XRT position (Beardmore et al.
GCN Circ. 20887) is detected in the initial UVOT exposures. The GRB
afterglow was also observed by Mazaeva et al. (GCN Circ. 20886) and
Mao et al. (GCN. Circ. 20889) in the R band.

The preliminary UVOT position is:
    RA  (J2000) =  06:12:14.82 =  93.06173 (deg.)
    Dec (J2000) = +50:29:35.2  =  50.49310 (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               87          237          147         17.53 +/- 0.04
v                  629         1597          117         18.36 +/- 0.19
b                  554         1349           78         19.12 +/- 0.19
u                  299          549          246         17.75 +/- 0.07
w1                 678         7320          588         20.20 +/- 0.29
m2                 653         7115          398        >19.8
w2                 604         8141          607        >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.19 in the direction of the burst
(Schlegel et al. 1998).