GCN Circular 21188

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 170531B
152 s after the BAT trigger (Evans et al., GCN Circ. 21171
Not found
 
 
).
We detect a possible afterglow consistent with the position given by GTC
(de Ugarte Postigo et al., GCN Circ. 21177
Not found
 
 
) and MASTER (Tyurin et al., 
GCN CIrc. 21179
Not found
 
 
).

The preliminary UVOT position is:
    RA  (J2000) =  19:07:32.10 = 286.88373 (deg.)
    Dec (J2000) = -16:25:05.8  = -16.41829 (deg.)
with an estimated uncertainty of 0.1 arc sec. (radius, 90% confidence).

This position is 3.0 arc sec away from the enhanced XRT position
(Osborne et al. GCN Circ. 21175
Not found
 
 
). Given the fading, detection
by other observatories and lack of matching source in the DSS, we find it
likely to be the GRB afterglow.

Preliminary 3-sigma magnitudes 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)         152          302          147         20.50+-0.29
white              590         1532          244         >21.03
v                  640         1581          116         >18.88
b                  566         1508           97         >19.77
u (fc)             310          560          245         >20.07
u                  713         1643           85         >19.17
uvw1               689         1630           97         >18.91
uvm2               664         1606          116         >18.88
uvw2               615         1557          116         >19.14

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.19 in the direction of the burst
(Schlegel et al. 1998).