GCN Circular 18084

A. A. Breeveld (UCL-MSSL) and S. B. Cenko (GSFC) report on behalf of the 
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150727A
86 s after the BAT trigger (Cenko et al., GCN Circ. 18076
Not found
 
 
).
A fading source consistent with the enhanced XRT position (Osborne et 
al. GCN Circ. 18079
Not found
 
 
) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  13:35:52.52 = 203.96885 (deg.)
     Dec (J2000) = -18:19:31.3  = -18.32536 (deg.)
with an estimated uncertainty of 0.44 arc sec. (radius, 90% confidence).

Preliminary detections in the white, u and uvw2 filters 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          236          147         18.28 �� 0.06
white              578          598           20         18.75 �� 0.24
v                 4036         4236          197        >18.7
b                  554          574           19        >18.5
u                  298          548          246         18.75 �� 0.16
m2                4241         4379          136        >19.1
w2                3831         4031          197         19.53 �� 0.36

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening in the direction of the burst.