GCN Circular 21184

M. H. Siegel (PSU) and P. A. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170531A
3523 s after the BAT trigger (Evans et al., GCN Circ. 21170
Not found
 
 
).
No optical afterglow consistent with the XRT position
(Melandri et al., GCN Circ. 21180
Not found
 
 
) is detected in the initial UVOT exposures.
We note that the XRT position is 3.5� away from a known USNO-B1 source, which
we detect. There is no indication of a second source within the error circle.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

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

white_FC          3523         3673          147         >18.1
white             3523         4292          344         >18.2
v                 4502         4702          197         >17.5
b                 3887         4087          197         >18.2
u                 3682         3881          197         >18.0
w2                4298         4497          197         >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.09 in the direction of the burst
(Schlegel et al. 1998).