TITLE:   GCN CIRCULAR
NUMBER:  40509
SUBJECT: GRB 250520A: Swift/UVOT Upper Limits
DATE:    25/05/21 13:02:53 GMT
FROM:    Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk>

S. R. Oates (Lancaster U.) and R. A. J. Eyles-Ferris (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250520A
127 s after the BAT trigger (Eyles-Ferris et al., GCN Circ. 40491).
No optical afterglow consistent with the enhanced XRT position (Goad et al., GCN Circ. 40494)
or the SVOM/VT optical candidate (Xin et al., GCN Circ. 40500) is detected in
the initial UVOT exposures.

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/3sigUL

white        149         298      147        >20.9
white        149         1373     392        >21.3
     v        127         1251     110        >18.9
     b        481         1349      97        >19.7
     u        456         1324      97        >19.3
uvw1        432         1300      97        >19.1
uvm2        407         1275      78        >18.6
uvw2        358         1394     112        >19.2

We caution that the photometry may not be accurate, as the source region
used to determine the optical/UV limits, is on an area of the UVOT
detector with low sensitivity. The magnitudes in the table are not corrected
for the Galactic extinction due to the reddening of E(B-V) = 0.754 in the
direction of the burst (Schlegel et al. 1998).