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. View this GCN Circular online at https://gcn.nasa.gov/circulars/36940. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...