TITLE:   GCN CIRCULAR
NUMBER:  29169
SUBJECT: GRB 201223A: NOT optical upper limit and NEXT powerlaw decay
DATE:    20/12/24 14:29:07 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

D. Xu (NAOC), D. B. Malesani (DTU Space), P. Galindo (NOT), Z.P. Zhu 
(NAOC, HUST), D. A. Kann (HETH/IAA-CSIC), report on behalf of a larger 
collaboration:

We observed the field of GRB 201223A (Gropp et al., GCN 29158) using the 
2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. 
We obtained 2x120s Sloan r-band frames starting at 02:21:52 UT on 
2020-12-24, i.e., 8.4 hr after the BAT trigger.

The previously reported optical afterglow (Lipunov et al. GCN 29157; 
Gropp et al., GCN 29158; Zhu et al. GCN 29159; Hu et al. GCN 29160; 
Hosokawa et al. GCN 29164; Romanov et al. GCN 29165; Xin et al. GCN 
29166; Belkin et al., GCN 29167; Kumar et al., GCN 29168) is not 
detected in the stacked image, down to a limiting magnitude of r > 22.0 mag.

Further analysis of our previous NEXT observations (Zhu et al. GCN 
29159) shows that the afterglow is decaying as F_rband ~ t^(-\alpha), 
where \alpha = 1.1 since the very beginning. The afterglow would be 
slightly fainter than 22.0 mag if this powerlaw decay is extrapolated 
until the NOT observational time, being consistent with the observed NOT 
upper limit. So the afterglow could be decaying in a single powerlaw 
until the NOT observation or have broken to a steeper decay before the 
NOT observation.