TITLE:   GCN CIRCULAR
NUMBER:  42319
SUBJECT: GRB 251013C: JinShan observations of the non-thermal rebrightening of the optical counterpart
DATE:    25/10/17 05:58:17 GMT
FROM:    L. B. He at NAOC <helb@bao.ac.cn>

L.B. He, X. Liu, J. An, S.Q. Jiang, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu, A.D. Zhu, L. Lei, H.Z. Wu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration:

We observed the field of GRB 251013C (Fermi GBM team, GCN 42221; Rakotondrainibe et al., GCN 42222) using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Observations started at 2025-10-16 13:23:15 UT, i. e., 2.822 days after the trigger, and a series of 300s frames in g, r, i, and z bands were obtained.

The previously reported optical counterpart (e.g., Palmerio et al., GCN 42223; and many late GCNs) was clearly detected in all the four bands.

Our preliminary analysis gives r = 21.04 +/- 0.06 at a median time 69 hrs post-burst. The magnitude was calibrated with nearby Pan-STARRS1 dr2 catalog and without the Galactic extinction correction, being consistent with the rebrightening reported in Méndez et al. (GCN 42318).

The SED from the g/r/i/z bands is non-thermal, being consistent with the behavior of GRB optical afterglows. Also a SN1998bw-like supernova would be much fainter at this redshift (z=0.572). 

We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.