TITLE:   GCN CIRCULAR
NUMBER:  36940
SUBJECT: GRB 240727A: TRT optical afterglow detection
DATE:    24/07/27 05:10:51 GMT
FROM:    Dong Xu at NAOC/CAS <dxu@nao.cas.cn>

X. Liu, S.Q. Jiang (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), J. An, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang (NAOC) report on behalf of a large collaboration:

We observed the field of GRB 240727A detected by Swift (Parsotan et al., GCN 36938), using the 0.7-m telescope of the Thai Robotic Telescope (TRT) network, located at Cerro Tololo, Chile. Observations started at 04:04:58 UT on 2024-07-27 and 4 x 300 s frames were obtained in the R-band.

An uncatalogued and fading source is detected in each frame within the Swift/XRT error circle and localized at coordinates

R.A. (J2000) = 15:45:45.01
Dec. (J2000) = +8:37:12.61

with an uncertainty of ~ 0.5 arcsec, being positionally consistent with the KAIT measurement (Zheng & Filippenko, GCN 36939) . The source had R = 18.6 +/- 0.1 mag in the first frame and faded to R = 19.2 +/- 0.1 mag in the last frame, calibrated with PanSTARRS and not corrected for Galactic extinction.

We therefore conclude that the source is the optical afterglow of GRB 240727A.