TITLE:   GCN CIRCULAR
NUMBER:  41471
SUBJECT: GRB 250821A: Swift/UVOT Upper Limits
DATE:    25/08/21 17:55:04 GMT
FROM:    Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk>

S. R. Oates (Lancaster U.) and S. Dichiara (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250821A
147 s after the BAT trigger (Dichiara et al., GCN Circ. 41462).
No optical afterglow consistent with the XRT position
(Evans et al., GCN Circ. 41463)
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 (FC)        147          297           148           >20.8
   u (FC)        455          475           19            >18.3
white             147          1715          281           >21.1
   v             381          1593          136           >19.1
   b             480          1692          127           >19.9
   u             455          1667          136           >19.6
uvw1             431          1643          58            >18.7
uvm2             406          1618          136           >19.1
uvw2             357          1569          136           >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.015 in the direction of the burst
(Schlegel et al. 1998).