TITLE:   GCN CIRCULAR
NUMBER:  22674
SUBJECT: GRB 180425A: Swift/UVOT Upper Limits
DATE:    18/04/25 14:10:33 GMT
FROM:    Sam Emery at MSSL-UCL  <samuel.emery.15@ucl.ac.uk>

S.W.K. Emery (UCL-MSSL) and F. E. Marshall (NASA/GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180425A
88 s after the BAT trigger (Marshall et al., GCN Circ. 22672).
No optical afterglow consistent with the XRT position
(Osborne et al., GCN Circ. 22673)
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 early exposures are:

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

white               88         1201          353         >21.0
u                  302         1152          285         >20.5
v                  631         1250           78         >18.5
b                  557         1176           58         >19.4
uvw1               680         1299           78         >19.0
uvm2               827         1275           58         >18.5
uvw2               606         1226           58         >19.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.03 in the direction of the burst
(Schlegel et al. 1998).