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=&endDate=)) 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?rlkey=tkwx0xqhfs9vz3j3a6yn80wtn&st=56irvpkv&dl=0)

[GRB 251013C - Filter-r - 15 Oct](https://www.dropbox.com/scl/fi/dx9iwokwdc2k2s2bqjz2z/grb251013C15oct.jpeg?rlkey=tec4uclokgmdt63nd3rfyd4d4&st=7h2vd1ha&dl=0)

**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.