TITLE:   GCN CIRCULAR
NUMBER:  29149
SUBJECT: GRB 201221A: REM early-time afterglow detection
DATE:    20/12/23 14:46:01 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <pda.davanzo@gmail.com>

P. D'Avanzo, S. Covino, D. Fugazza, A. Melandri (INAF-OAB), D. B. Malesani (DTU space) 
on behalf of the REM team, report:

We observed the field of GRB 201221A (Page et al., GCN Circ. 29096) with the REM 60cm robotic 
telescope located at the ESO premise of La Silla (Chile). The observations were performed starting 
on 2020 December 21 at 07:10:33 UT (i.e. 92 seconds after the burst) and were carried in the g, r, i, z 
bands simultaneously. 

From preliminary photometry, we estimate the following magnitudes and 3sigma upper limits for the 
afterglow reported by Malesani et al. (GCN Circ. 29100):

g > 17.9
r > 17.8
i > 17.7
z = 15.8 +/- 0.3

(AB; calibrated against the APASS catalogues), at a mean time of 97 seconds after the GRB t0. 
By combining our z-band magnitude with the one reported by Malesani et al., we infer an afterglow 
temporal decay (f propto t^(alpha)) of alpha ~ -1.2 between 97 seconds and 2100 seconds after the GRB t0.