GCN Circular 19764

A. A. Breeveld (UCL-MSSL) and F. E. Marshall (NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160804A 82 
s after the BAT trigger (Marshall et al., GCN Circ. 19761
Not found
 
 
).
A fading source consistent with the XRT position (Osborne et al. GCN 
Circ. 19762
Not found
 
 
) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  14:46:31.14 = 221.62976 (deg.)
     Dec (J2000) = +09:59:56.7  =   9.99908 (deg.)
with an estimated uncertainty of 0.44 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               82          232          147         18.56 �� 0.06
white              794          944          148         19.66 �� 0.12
v                  570         6762          373        >19.9
b                  496         6227          432         20.4  �� 0.24
u                  240          490          246         19.53 �� 0.20
w1                4182         4382          197         18.90 �� 0.25
m2                1000         5612          216        >18.7
w2                 546         6637          432        >19.5

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.03 in the direction of the 
burst (Schlegel et al. 1998).