TITLE:   GCN CIRCULAR
NUMBER:  44363
SUBJECT: GRB 260421B: TRT optical counterpart detection
DATE:    26/04/21 05:26:32 GMT
FROM:    L. B. He at NAOC <helb@bao.ac.cn>

L.B. He (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), X. Liu, Z.P. Zhu, S.Q. Jiang, J. An, D. Xu (NAOC) report:

We observed the field of GRB 260421B detected by Fermi (GCN 44360) and SVOM (zhao et al., GCN 44362), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile (CTO).  We obtained a series of frames in the r-band. 

An uncatalogued source is detected within the Fermi error circle at coordinates
R.A. (J2000) = 12:40:00.5
Dec. (J2000) = -17:44:24.5
with an uncertainty of ~ 1.0 arcsec. The source had r = 19.14 +/- 0.07  mag, calibrated with Pan-STARRS DR2 and not corrected for Galactic extinction.

No NEO would be at the above position at the observational time by checking MPC.

We thus think the source is the optical counterpart of GRB 260421B.