GCN Circular 30256

N. P. M. Kuin (UCL-MSSL) and M. G. Bernardini (INAF-OAB) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210618A
3728 s after the BAT trigger (Bernardini et al., GCN Circ. 30250
Not found
 
 
).
No optical afterglow consistent with the XRT candidates source 1, 2
or 4 (Osborne et al. , GCN Circ. 30252
Not found
 
 
) 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          3728         3878          147         >20.4
white             3728         4496          344         >21.1
v                 4707         4843          133         >19.2
b                 4092         4291          197         >20.5
u                 3886         4086          197         >20.0
w2                4502         4702          197         >20.1

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).

Since this is a short GRB, further Swift search for late time optical
emission is being planned.