TITLE:   GCN CIRCULAR
NUMBER:  21184
SUBJECT: GRB 170531A: Swift/UVOT Upper Limits
DATE:    17/06/01 16:02:44 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <siegel@swift.psu.edu>

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).
No optical afterglow consistent with the XRT position
(Melandri et al., GCN Circ. 21180) 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).