TITLE: GCN CIRCULAR NUMBER: 42025 SUBJECT: GRB 250925A: Mondy I-band optical observations DATE: 25/09/29 13:09:19 GMT FROM: Nicolai Pankov at HSE, IKI RAS colinsergesen@gmail.com

N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) 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) starting on 2025-09-27 at 14:45 UT, i.e. ~1.93 days since trigger. In total we obtained 45x120 s exposures in the I-band using CMOS photometer. 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; Pankov et. al, GCN 42005) is not visible in the stacked image. Preliminary photometry and observation properties are presented below:

Date UTstart Exptime t-T0 Filter OT Err. UL FWHM (nxs) (mid, days) (3sigma) (") 2025-09-27 14:45:07 45x120 1.93451 I n/d n/d 22.3 1.6

The photometry was calibrated against nearby USNO-B1.0 stars (I-mags) and not corrected for the Galactic extinction.

View this GCN Circular online at https://gcn.nasa.gov/circulars/42025. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...