GCN Circular 29862

A. A. Breeveld (UCL-MSSL) and M. J. Moss (GWU) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210420B 147s after the BAT trigger 
(Moss et al., GCN Circ. 29844 and updated in Lien et al., GCN Circ. 29854).
A fading source consistent with the optical counterpart reported by Lipunov et al. GCN Circ. 29845, 
Oksanen GCN Circ. 29846, Hu et al. GCN Circ. 29847, Tianrui et al. GCN Circ. 29848, Leonini et al. 
GCN Circ. 29851, de Ugarte Postigo et al. GCN Circ. 29852 and Watson et al. GCN Circ. 29858, is 
detected in the initial UVOT exposures.

Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 
1358, 373) for the first finding chart (_fc) exposure and other early exposures are:

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

u_fc               147          391          240         16.2 �� 0.04
u                18793        19317          512         18.6 �� 0.1
v                  447          467           20         16.7 �� 0.2
b                  397          417           19         17.1 �� 0.1
w1                 496          516           20         16.5 �� 0.2
m2                 620         1121           39         17.2 �� 0.2
w2                1201         1371           39         17.6 �� 0.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.022 in the direction of the burst (Schlegel et al. 1998).