GCN Circular 14424

S. R. Oates, M. De Pasquale, and M. J. Page (UCL-MSSL)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 130420A
739 s after the BAT trigger (Page et al., GCN Circ. 14406
Not found
 
 
).
A source consistent with the XRT position
(Goad et al. GCN Circ. 14416
Not found
 
 
) is detected in the initial UVOT exposures.
The GRB is detected in all filters and is therefore consistent with
being at a redshift of  ~<1.6.

The preliminary UVOT position is:
  RA  (J2000) =  13:04:25.55 = 196.10647 (deg.)
  Dec (J2000) = +59:25:27.1  =  59.42420 (deg.)
with an estimated uncertainty of 0.50 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              739          888          147         17.14 +/- 0.03
v                 1072         1092           20         17.66 +/- 0.31
b                 1171         1191           19         18.13 +/- 0.21
u                 1146         1166           20         17.19 +/- 0.17
w1                1122         1141           19         17.51 +/- 0.29
m2                1097         1465           58         18.30 +/- 0.31
w2                1049         1240           39         18.50 +/- 0.33

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).