GCN Circular 19530

M. De Pasquale (UCL-MSSL) and D. Malesani (DARK/NBI)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160611A
2687 s after the BAT trigger (Malesani et al., GCN Circ. 19522
Not found
 
 
).

Very close to the XRT error circle (Osborne et al. GCN Circ. 19523
Not found
 
 
), two
sources are detected in the UVOT images. These sources are also detected
in the Digitized Sky Survey (DSS) and are listed in the USNO catalog.
Their light would blend with any new source at the XRT position, making
the analysis more delicate.

None of the two sources shows any significant flux decrease in the white
filter between the first two white exposures (mid-time: 3071 s after
the trigger) and the last three white exposures (mid-time: 10030 s after
the trigger). Similarly, we find no significant decrease of the flux
of the two sources in the u filter between the first u filter exposure
(mid-time: 2946 s) and the last one (mid-time: 8560 s).
We conclude that no optical afterglow consistent with the XRT position
is detected in the UVOT exposures.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white             2687         3456          344         >19.6
white             9578        10478          880         >19.8
v                 3668         3867          197         >18.1
b                 3051         9573         1082         >18.9
u                 2846         8660          393         >19.6
w1                4078         5167          191         >19.7
m2                3873         4072          197         >19.4
w2                3463        10905          611         >20.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.22 in the direction of the burst
(Schlegel et al. 1998).