GCN Circular 20894

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
Not found
 
 
).
A fading source consistent with the XRT position (Beardmore et al.
GCN Circ. 20887
Not found
 
 
) is detected in the initial UVOT exposures. The GRB
afterglow was also observed by Mazaeva et al. (GCN Circ. 20886
Not found
 
 
) 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).