TITLE:   GCN CIRCULAR
NUMBER:  28743
SUBJECT: GRB 201020B: Swift/UVOT Detection
DATE:    20/10/22 12:34:19 GMT
FROM:    Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>

A. A. Breeveld (UCL-MSSL) and A. Y. Lien (GSFC/UMBC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 201020B 72102 s after the Fermi 
GMB/LAT trigger (Fermi GBM team, GCN Circ. 28702; Malacaria and Meegan, GCN Circ. 28710; Arimoto et 
al., GCN Circ. 28716).
A source consistent with the Master position (Lipunov et al., GCN Circ. 28718), also detected by Xu 
et al., (GCN Circ. 28719), Ridnaia et al. (GCN Circ. 28723), Belkin et al. (GCN Circ. 28723) and 
Sbarufatti et al. (GCN Circ. 28740), is detected in the initial UVOT exposures. We note there is a 
faint USNO-B1.0 source (1670-0037526, Bmag ~21.7) 5.2" away from this position.

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            73636        73973          332         19.54 ± 0.09
v                73979        74316          331         19.30 ± 0.28
u                72102        72821          708         19.12 ± 0.11
u		 73293	      73631	     332	 19.41 ± 0.19

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.15 in the direction of the burst (Schlegel et al. 1998).