TITLE: GCN CIRCULAR NUMBER: 41930 SUBJECT: GRB 250917B: Mondy and Zeiss-1000 SAO RAS optical observations, optical observations, afterglow candidate DATE: 25/09/21 13:38:53 GMT FROM: Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com> N. Pankov (HSE), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), E. Klunko (ISTP), O. Spiridonova (SAO RAS) report on behalf of GRB IKI FuN: We observed the field of Swift XRT of GRB 250917B (Fermi GBM team GCN 41846; Caputo et al. GCN 41848 ) with AZT-33IK telescope of Mondy observatory in R-filter on 2025-09-17 starting (UT) 13:36:18(epoch 1) and Zeiss-1000 of SAO RAS starting on 2025-09-17 (UT) 16:24:04 (epoch 2). We do not detect any new sources within S1, S5, S6 XRT localisation (Kennea et al., GCN 41851). This is consistent with optical observations (Hu et al, GCN 41854; Breeveld et al, GCN 41864; Yadav et al., GCN 41872; Zheng et al., GCN 41927). However, at the boundary of the S5 XRT circle, we found an object (coordinates are (J2000) 19:25:33.74 +11:04:24.85) that was significantly brighter in epoch 1 compared to epoch 2. The photometry of the object is the following Date UT start t-T0 Filter OT Err. UL(3sigma) Telescope (mid, days) (s) 2025-09-17 13:36:18 0.31845 R 19.97 +/- 0.08 21.4 AZT-33IK 2025-09-17 16:24:04 0.42394 Rc 20.79 +/- 0.12 21.3 Zeiss-1000 We suggest the source is an afterglow candidate of GRB 250917B. The object is a diffuse one and most likely is the galaxy. View this GCN Circular online at https://gcn.nasa.gov/circulars/41930. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...