TITLE:   GCN CIRCULAR
NUMBER:  27971
SUBJECT: GRB 200613A: Jet-break from Xinglong-2.16m optical observations
DATE:    20/06/17 01:39:34 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

S.Y. Fu, Z.P. Zhu, X. Liu, D. Xu, J. Zheng, A.Y. Zhou (NAOC) report:

We continued to observe the field of GRB 200613A (Fermi GBM team, GCN 
27926; Bissaldi et al., GCN 27930; Ohno et al., GCN 27931) using the 
Xinglong-2.16m telescope equipped with the BFOSC camera. Observations 
were carried out starting at 12:55:02 UT on 2020-06-16, and 5x300s 
R-band frames were obtained.

The optical afterglow (e.g., Kann et al., GCN 27935; Kennea et al., GCN 
27936; Pozanenko et al., GCN 27937; Izzo et al., GCN 27939; Zhu et al., 
GCN 27943; Marshall, GCN 27944; Liu et al., GCN 27949; Belkin et al., 
GCN 27958) is still well detected in our stacked image, with R = 21.07 
+/- 0.21 mag (at Tmid-T0 = 3.3175 days post-burst), calibrated with 
nearby Pan-STARRS1.

Compared with our previous measurements, the afterglow now has turned to 
decay quickly, with a temporal decay index of  \alpha ~ 2.5 and a break 
time of ~ 2.3 days post-burst. This break can be interpreted as a jet break.

Further observations are planned.