GCN Circular 41416
Subject
GRB 250818A: Swift/UVOT Upper Limits
Event
Date
2025-08-18T17:50:17Z (7 days ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250818A
162 s after the BAT trigger (Cenko et al., GCN Circ. 41403).
No optical afterglow consistent with the XRT position (Osborn et al., GCN
Circ. 41403) 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
u (fc) 162 412 246 >20.3
b 418 438 19 >18.6
v 32212 32641 429. >20.2
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.602 in the direction of the burst
(Schlegel et al. 1998).