TITLE: GCN CIRCULAR NUMBER: 42736 SUBJECT: GRB 251013C: Koshka, SAO RAS, Mondy, and CrAO optical observations, possible SN detection DATE: 25/11/18 15:46:36 GMT FROM: Alina Volnova at IKI RAS <alinusss@gmail.com> A. Volnova (IKI), A. Pozanenko (IKI), A. S. Moskvitin (SAO RAS), V. Rumyantsev (CrAO), A. Novichonok (Petrozavodsk State University, KIAM), E. Klunko (ISTP), O. I. Spiridonova (SAO RAS), T. A. Fatkhullin (SAO RAS) report on behalf of IKI-GRB-FuN: We observed the optical afterglow (Palmerio et al., GCN 42223; Perez-Garcia et al., GCN 42225; Konno et al., GCN 42226; Martin-Carrillo et al., GCN 42227; Masi, GCN 42228; Palmerio et al., GCN 42229; Moskvitin et al., GCN 42230; Gompertz et al., GCN 42231; Lipunov et al., GCN 42235; Garnavich, GCN 42240; Watson et al., GCN 42241; López-Cámara et al., GCN 42242; Brosio et al., GCN 42251; Reguitti, GCN 42253; Quintana-Ansaldo et al., GCN 42254; Maksut et al., GCN 42256; Ghosh et al., GCN 42261; Quadri et al., GCN 42262; Adami et al., GCNs 42266, 42276, 42338; Peretto et al., GCN 42267; Ruocco et al., GCN 42269; Calapai, GCN 42275; Globus et al., GCN 42279; Evans, GCN 42281; Abidkhanov et al., GCN 42283; Siegel, GCN 42298; Leonini et al, GCN 42301; Méndez et al., GCN 42318; He et al., GCN 42319; Pavoni et al., GCN 42333; Bochenek and Perley, GCN 42334; Volnova et al., GCN 42396; Moskvitin et al., GCN 42399; Khosroshahi et al., GCN 42492; Hong et al., GCN 42569) of the GRB 251013C (The Fermi GBM team, GCN 42221; Rakotondrainibe et al., GCN 42222) with four facilities of the IKI GRB-FuN network: the 1-meter Zeiss-1000 telescope of the Koshka Observatory (INASAN) on Oct. 15 and 20, 2025, the 1.5-meter AZT-33IK telescope of Sayan Solar Observatory (Mondy) on Oct. 18-19, 21-23, 26-28, 2025, the 1-m SAO RAS telescope Zeiss-1000 equipped with the CCD-photometer on Oct. 17-18, 20, 24-25, 28, 2025, and the ZTSh 2.6m telescope at the CrAO observatory on Oct. 25, 2025. The preliminary photometry and observational details are the following: Date UT start t-T0 Exp. Filter OT Err. UL FWHM," Observatory/Scope (mid,days) (n*s) (3sigma) 2025-10-15 17:37:25 2.07846 116*90 R n/d n/d 21.5 3.5 Koshka/Ziess-1000 2025-10-17 16:59:44 3.99336 10*300 Rc 20.41 0.03 23.0 3.9 SAO/Zeiss-1000 2025-10-18 14:42:05 4.89113 2*120 R 20.82 0.15 21.5 2.0 Mondy/AZT-33IK 2025-10-18 18:54:43 5.08147 10*300 Rc 21.02 0.04 23.4 2.9 SAO/Zeiss-1000 2025-10-19 12:36:06 5.81400 35*120 R 21.58 0.06 23.2 1.7 Mondy/AZT-33IK 2025-10-20 18:43:33 7.08815 40*180 R 20.70 0.14 21.8 2.6 Koshka/Ziess-1000 2025-10-20 19:59:38 7.12682 13*300 Rc 21.61 0.11 22.7 5.4 SAO/Zeiss-1000 2025-10-21 12:30:55 7.81336 40*120 R 21.89 0.09 23.0 2.0 Mondy/AZT-33IK 2025-10-22 12:29:59 8.85439 37*120 R 21.63 0.12 22.4 2.1 Mondy/AZT-33IK 2025-10-23 12:05:16 9.79207 35*120 R 22.31 0.22 22.5 2.7 Mondy/AZT-33IK 2025-10-23 19:20:37 10.08190 6*300 Rc 22.31 0.25 22.5 2.3 SAO/Zeiss-1000 2025-10-24 18:54:05 11.06288 6*300 Rc n/d n/d 22.0 2.4 SAO/Zeiss-1000 2025-10-25 16:11:31 11.97093 38*120 R 21.70 0.08 23.1 2.5 CrAO/ZTSh 2025-10-26 13:33:51 12.86054 45*120 R 21.70 0.10 22.9 2.2 Mondy/AZT-33IK 2025-10-27 12:22:47 13.81189 45*120 R 22.13 0.15 22.7 2.6 Mondy/AZT-33IK 2025-10-28 12:32:36 14.81800 44*120 R 22.23 0.25 22.4 3.6 Mondy/AZT-33IK 2025-10-28 19:37:11 15.11907 10*300 Rc 22.46 0.22 22.8 3.6 SAO/Zeiss-1000 The photometry is based on several nearby SDSS stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction; all magnitudes are in Vega system. The light curve based on our previous and presented above data can be found at https://heaiki.ru/lvc/r/PD/GRB251013C/GRB251013C_LC.jpg There are at least three bumps that can be seen in the LC, the first re-brightening already mentioned earlier (Méndez et al., GCN 42318; Adami et al., GCN 42338). The second bump is on the 9th day, and the 3rd bump has the maximum between 12 and 13 days. The last bump could be related to a supernova associated with GRB 251013C. Given the redshift of the gamma-ray burst source z = 0.572 (Martin-Carrillo et al., GCN 42227) maximum in the LC on days 12–13 may be a supernova associated with GRB 251013C. If so, the absolute magnitude of the SN at its maximum is M_R > -20.3, which is consistent with the distribution of absolute maxima of SNe associated with GRBs (see, e.g., Belkin et al., 2024). View this GCN Circular online at https://gcn.nasa.gov/circulars/42736. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...