TITLE: GCN CIRCULAR NUMBER: 41892 SUBJECT: GRB 250916A: Mondy R-band optical observations DATE: 25/09/19 20:44:29 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) A. Moskvitin (SAO RAS) report on behalf of IKI-GRB-FuN: We observed the field of a long GRB 250916A at the redshift z = 2.015 (de Ugarte Postigo et al., GCN 41863) detected by Fermi/GBM (The Fermi GBM team, GCN 41839), AstroSat/CZTI (Arya et al., GCN 41843), Glowbug (Cheung et al., GCN 41855), Swift/XRT (Osborne et. al, GCN 41862), and NuSTAR (Waratkar et. al, GCN 41871). The observations were carried with the AZT-33IK 1.5m reflector of Sayan Solar Observatory (Mondy) equipped with the CMOS-photometer. The series of 25x120 s images was obtained in the R-band starting on 2025-09-18 at 18:36 UT, i.e. ~2.23 days since GRB. In the stacked image we clearly detect the OT found initially by GOTO (Belkin et al., GCN 41847) and followed-up by (Mohan et al., GCN 41858; Moskvitin et. al, GCN 41859; Burkhonov et al., GCN 41860; Moskvitin et. al, GCN 41870; Zheng & Filippenko, GCN 41873). Preliminary observation results are presented below: Date UT start t-T0 Exp. Filter OT Err. UL (mid, days) (n*s) (3sigma) 2025-09-18 18:36:27 2.23063 25x120 R 20.52 0.02 21.5 The photometry is based on nearby stars of PanSTARRS-DR1 (R-magnitudes were obtained via the Lupton 2005 transformations; see Moskvitin et. al, GCN 41859) and not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41892. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...