TITLE:   GCN CIRCULAR
NUMBER:  33683
SUBJECT: GRB 230423A: Swift/UVOT Upper Limits
DATE:    23/04/25 20:34:48 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

N. Paul Kuin (UCL-MSSL) and Riccardo Brivio (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230423A
731 s after the BAT trigger (Brivio et al., GCN Circ. 33677).  No optical
afterglow consistent with the XRT position  Salvaggio et al., GCN
Circulars 33679,  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

white_FC           731          881          147         >20.9
white              731         5415          491         >21.4
b                 5010         5210          197         >20.0
u                 4805         5004          197         >19.2
w2                5420         5544          122         >19.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.137 in the direction of the burst
(Schlegel et al. 1998).