TITLE:   GCN CIRCULAR
NUMBER:  31098
SUBJECT: GRB 211023A: optical observations and photometric evidence of supernova
DATE:    21/11/20 14:54:34 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

S. Belkin (IKI), A. Pozanenko (IKI), I. Sokolov (INASAN, KIAM), E. 
Klunko (ISTP), A. S. Moskvitin (SAO RAS), V. Kim (HSE, FAI), V. 
Rumyantsev (CrAO),  R. Ya. Inasaridze (AbAO),  M. Krugov (FAI), D. 
Berezin (IC AMER NASU), G. Butenko (IC AMER NASU), N. Pankov (HSE), D. 
Datashvili (AbAO), V. R. Ayvazian (AbAO),  G. V. Kapanadze (AbAO) report 
  on behalf of GRB IKI FuN:

We observed the field of the GRB 211023A (Fermi GBM team, GCN 30958; Di
Lalla et al., GCN 30961; Lesage et al, GCN 30965; Ursi et al, GCN 30969;
Poolakkil, GCN 30998; Ridnaia et al., GCN 31022) with Zeiss-2000 and
Zeiss-600 telescopes of Terskol observatory, AZT-33IK telescope of Sayan
observatory (Mondy), AZT-20 telescope of Assy-Turgen observatory, ZTSh
2.6m-telescope of CrAO observatory, AS-32 telescope of AbAO observatory,
and Zeiss-1000 1-m telescope of SAO RAS. The optical counterpart
(Lipunov et al., GCN 30970; Zhirkov et al, GCN 30977; Kann et al, GCN
30982; Hu et al, GCN 30990; Belkin et al, GCN 31004; Vinko et al, GCN
31008; Belkin et al, GCN 31018; Belkin et al, GCN 31020; Kumar et al,
GCN 31023; Gupta et al., GCN 31041; Strobl et al., GCN 31042; Pozanenko
et al., GCN 31053; Moskvitin et al., GCN 31066) was detected in most of
the stacked images. A preliminary light curve based on our observations 
and GCN Circulars cited above can cab be found at

http://grb.rssi.ru/GRB211023A/GRB211023A_LC.png

Starting from the fifth day, a systematic flux excess over the
approximation of the afterglow by a single power law is observed. After 
subtraction of the afterglow approximated by the light curve with
a power law (Belkin et al, GCN 31020), and neglecting a contribution of
the host galaxy we determine a preliminary light curve of possible SN, see

http://grb.rssi.ru/GRB211023A/GRB211023A_AG_subtracted_LC.png

Using redshift of GRB 211023A (Pozanenko et al., GCN 31053)
we calculated an absolute magnitude at the maximum of the supposed
supernova (M_R=-19.9 +0.40 -0.29; adjusted by a Galactic extinction
(Schlafly & Finkbeiner, 2011)) and a time since the burst trigger in
the observer frame (t-T0 = 16.9+/-1.5). These parameters are in
agreement with parameters of known SN-GRB (e.g. see Figure 12 from 
Belkin et al., Astronomy Letters, 2020). The supernova associated with 
GRB 211012A is in agreement with early predictions (Kann et al, GCN 
30982; Aimuratov et al., GCN 31056; Minaev et al., GCN 31081).