TITLE: GCN CIRCULAR NUMBER: 42492 SUBJECT: GRB 251013C : 3.4-m telescope of Iranian National Observatory (INO340) optical observations DATE: 25/10/27 20:37:15 GMT FROM: soroush shakeri at Iranian National Observatory (INO) - Isfahan University of Technology (IUT) <soroush.physic@gmail.com> **Authors: INO340 Transient Follow-up Team** (H. G. Khosroshahi (INO), S. Shakeri (IUT/INO), H. Altafi (INO/IUT), S. Karimi Mianrudi (IUT), Parisa Hashemi (IUT), H. Torkzadeh (IUT), Reza Rezaei (SUT) and Yu Wang (ICRANet). We report optical follow-up observations of the long-duration **GRB 251013C**, detected by the Fermi/GBM team (GCN [42221](https://gcn.nasa.gov/circulars/42221?view=index&query=GRB+251013C&startDate=...)) at 2025-10-13 17:39:41 UT, and subsequently observed by SVOM and Swift/XRT. We observed the field of GRB 251013C with **3.4 m Telescope of Iranian National Observatory** ([INO340](https://ino.org.ir/en/)) by using SDSS g, r, and i filters on 2025 October 13, 15, and 16 (UT). We detected the source at the following position R.A. (J2000.0): 23 03 16 Dec.(J2000.0): -00 12 21 Our first observation was done at 20:36:20 UT on October 13, corresponding to ~10,578 s after the trigger of the Fermi/GBM, and the observation extended to ~2.17 days post-trigger. Photometric calibration was carried out using nearby reference stars from Pan-STARRS and SDSS catalogs. The afterglow shows a consistent fading trend typical of long GRB afterglows and a slight brightening in the second day, with magnitudes decreasing from i = 16.3 ± 0.018 → 21.61 ± 0.34, r = 16.63 ±0.20 → 21.99 ± 0.25, and g = 16.75 ± 0.017→ 22.44 ± 0.16. The deepest stacked images (exp:500s) from the third night reached g = 22.44 ± 0.16 mag. The optical images [ GRB 251013C - Filter-r - 13 Oct](https://www.dropbox.com/scl/fi/5104th21jqrydd9box07q/grb251013C13oct.jpeg?rl...) [GRB 251013C - Filter-r - 15 Oct](https://www.dropbox.com/scl/fi/dx9iwokwdc2k2s2bqjz2z/grb251013C15oct.jpeg?rl...) **We thank the INO340 technical and scientific staff**, M. Bidar, A. Javadi, A. Farhang, H. Shalchian, R. Shomali, R. Ravanmehr, H. Jenab, S. M. Mohajer, H. Sadeghi, M. Rostamian, A. Behnam, A. Karimzadeh, T. Shokatpour, R. Mirzaei, H. Alirezaei, S. Masoudieh. View this GCN Circular online at https://gcn.nasa.gov/circulars/42492. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...