TITLE:   GCN CIRCULAR
NUMBER:  29109
SUBJECT: GRB 201221A: Swift/UVOT Upper Limits
DATE:    20/12/21 22:06:23 GMT
FROM:    Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>

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

The Swift/UVOT began settled observations of the field of GRB 201221A 141 s after the BAT trigger 
(Page et al., GCN Circ. 29096). No optical afterglow consistent with the optical position (Malesani 
et al. GCN Circ. 29100) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. 
Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:

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

white_FC           141          291          147         >20.0
u_FC               300          549          246         >19.1
white              141          951          257         >20.2
v                  629         4762          236         >18.8
b                  555          748           39         >18.3
u                  300         5271          357         >19.5
w1                 679         5172          236         >19.4
m2                 654         4967          236         >19.2
w2                 605         4557          235         >19.4

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.11 in the direction of the burst (Schlegel et al. 1998).