GCN Circular 22805

A. A. Breeveld (UCL-MSSL) and P. A. Evans (U. Leicester) report on 
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 180620A 
270 s after the BAT trigger (Evans et al., GCN Circ. 22798
Not found
 
 
).
A fading source consistent with the XRT position (Osborne et al. GCN 
Circ. 22802
Not found
 
 
), and also detected by Butler et al. (GCN Circ. 22799
Not found
 
 
), 
Elenin et al. (GCN Circ. 22800
Not found
 
 
), Tyurina et al. (GCN Circ. 22801
Not found
 
 
) and 
Guidorzi et al. (GCN Circ. 22803
Not found
 
 
), is detected in the initial UVOT 
exposures. The detection in the UVOT uvw2 filter implies a redshift 
limit of 1.2.

The preliminary UVOT position is:
     RA  (J2000) =  18:39:35.09 = 279.89619 (deg.)
     Dec (J2000) = +23:14:39.1  =  23.24419 (deg.)
with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence).

Preliminary detections 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              270          420          147         17.68 �� 0.04
v                  427         1420          117         18.43 �� 0.19
b                  525          717           39         18.77 �� 0.20
u                  500          692           39         17.92 �� 0.17
w1                 476         2158          194         18.86 �� 0.20
m2                 623         7111          391         19.65 �� 0.27
w2                 574         8136          588         20.00 �� 0.23

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.13 in the direction of the 
burst (Schlegel et al. 1998).