GCN Circular 23205

M. H. Siegel (PSU) and A. D'Ai (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180904A
131 s after the BAT trigger (D'Ai et al., GCN Circ. 23194
Not found
 
 
).
No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 23196
Not found
 
 
)
is detected in the initial UVOT exposures, consistent with the non-detections
of Malesani et al. (GCN Circ. 23195
Not found
 
 
), Izzy et al. (GCN Circ 23197), 
Moskvitin et al. (GCN Circ. 23198
Not found
 
 
) and Guidorzi et al. (GCN Circ. 23199
Not found
 
 
).
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           131          281          147         >21.1
u_FC               289          539          246         >20.1
white              131          589          167         >21.1
v                  620          640           19         >20.1
b                  545          565           19         >19.4
u                  289          539          246         >20.1
w1                 670          690           19         >18.1
w2                 595          615           19         >18.6

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).