TITLE:   GCN CIRCULAR
NUMBER:  31234
SUBJECT: GRB 211211A: SAO RAS optical observations
DATE:    21/12/15 17:28:29 GMT
FROM:    Moskvitin Alexander at SAO RAS  <mosk@sao.ru>

A. Moskvitin (SAO), O. Spiridonova (SAO), S. Belkin (IKI),
A. Pozanenko (IKI), N. Pankov (HSE, IKI)  report
on behalf of GRB IKI FuN:

We observed  the field of GRB 211211A (Fermi team GCN 31201,
D'Ai et al. GCN 31202; Mangan et al. GCN 31210; Tamura et al.
GCN 31226) with Zeiss-1000 telescope of SAO RAS in Rc-filter
on 2021.12.14 (UT) 02:03:33.

We marginally detected an afterglow (Zheng and Filippenko GCN 31203;
Jiang  et al. GCN 31213; Strausbaugh et al. GCN 31214; Ito  et al.
GCN 31217; de Ugarte Postigo et al. GCN 31218,31228,31229; Malesani et
al. GCN 31221; Belles et al. GCN 31222; Kumar et al. GCN 31227; Pankov
et al. GCN 31233).

Preliminary photometry of the afterglow is following

Date       UT start   t-T0    Filter Exp.   OT   Err.     UL(3sigma)
                        (mid, days)    (s)
2021-12-14 02:03:33   2.55803   Rc   6*600  23.2 S/N=2.5  23.1

The photometry is based on the nearby SDSS stars:
SDSS-DR12
RA          DEC          R(Lupton transformations)
14:09:08.84 +27:53:54.56 15.833
14:09:04.53 +27:54:10.48 16.701
14:08:57.03 +27:54:50.26 17.677
14:08:57.88 +27:55:06.09 17.805
14:09:03.81 +27:55:56.55 17.104
14:09:07.89 +27:55:44.00 18.640

The light curve based on published photometry can be found in
http://grb.rssi.ru/GRB211211A/GRB211211A_LC.png

Using R-photometry we found a single power law decay index of -1.5
between 0.4 and 2.6 days after the burst trigger. However the power
law index between 1.6 days (Pankov et al. GCN 31233) and 2.6 days
(this circ.) we may assume power law index of -3.25 which confirm
very fast decay in i-filter observed by CAFOS/2.2mCAHA
(de Ugarte Postigo et al. GCN 31229). Based on a typical power law
decay of afterglows we may suggest a bump in the LC at ~ 2.5 days
followed by a power law decay index of 1.5.