GCN Circular 22611

M. H. Siegel (PSU) and S. J. LaPorte (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180404B
82 s after the BAT trigger (LaPorte et al., GCN Circ. 22590
Not found
 
 
).
A fading source consistent with the XRT position
(Beardmore et al. GCN Circ. 22594
Not found
 
 
)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
   RA  (J2000) =  03:33:34.23 =  53.39262 (deg.)
   Dec (J2000) = -50:12:54.2  = -50.21506 (deg.)
with an estimated uncertainty of 0.46 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white (fc)          82          231          147         18.66 +/- 0.09
white               82         1720          411         19.39 +/- 0.09
white             6421         6620          196        >20.7
v                  624         7031          510        >19.5
b                  550         7661          322        >20.3
u (fc)             295          545          245         19.49 +/- 0.31
u                  295         1670          343         19.70 +/- 0.29
u                 6011         7648          393        >20.1
w1                 674         7442          510        >20.6
m2                1600         7236          413        >20.5
w2                 600         6826          452        >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).