TITLE: GCN CIRCULAR NUMBER: 42005 SUBJECT: GRB 250529A: Mondy and AbAO optical observations DATE: 25/09/26 15:27:34 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP), R. Ya. Inasaridze (AbAO) report on behalf of GRB-IKI-FuN: We observed the field of GRB 250925A detected by Swift (Dichiara et. al, GCN 41985) at z=3.899 (Castro-Tirado et. al, GCN 41997) with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) and the AS-32 0.7m telescope of the Abastumani Observatory (AbAO). The observations started at Mondy on 2025-09-25 at 20:00 UT, i.e. 0.118 days since trigger. In total we obtained 30x120 s exposures with AZT-33IK, and 85x60 s exposures with AS-32. The optical afterglow initially detected by NOT/ORM (Corcoran et. al, GCN 41990) and also observed by (Saikia et. al, GCN 41986; Pavoni & Moretti et. al, GCN 41989; Bochenek & Perley, GCN 41991; Masi et. al, GCN 41992; Mendez et. al, GCN 41995; Castro-Tirado et. al, GCN 41997; Liu et. al, GCN 41998; Ochi et. al, GCN 41999; Xin et. al., GCN 42002; Rakotondrainibe et. al, GCN 42004) is clearly visible in the stacked images from Mondy. Preliminary photometry is as follows: Date UTstart Exptime t-T0 Filter OT Err. UL Site/Telescope (nxs) (mid, days) (3sigma) 2025-09-25 20:00:15 6x120 0.12626 R 22.3 0.2 23.1 Mondy/AZT-33IK 2025-09-25 20:12:15 6x120 0.13460 R 22.1 0.2 23.0 Mondy/AZT-33IK 2025-09-25 20:24:16 6x120 0.14294 R 22.1 0.2 23.1 Mondy/AZT-33IK 2025-09-25 20:36:16 6x120 0.15128 R 22.1 0.1 23.1 Mondy/AZT-33IK 2025-09-25 20:48:16 6x120 0.15962 R 22.6 0.2 23.1 Mondy/AZT-33IK 2025-09-25 19:24:44 79x60 0.12486 R n/d n/d 21.7 AbAO/AS-32 The photometry was calibrated against nearby USNO-B1.0 stars (R2-mags) and not corrected for the Galactic extinction. Using all available r-data we estimate a temporal slope of alpha = -1.17 +/- 0.05. View this GCN Circular online at https://gcn.nasa.gov/circulars/42005. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...