TITLE: GCN CIRCULAR NUMBER: 43012 SUBJECT: GRB 251205A: Zeiss-1000 Koshka observatory optical observations DATE: 25/12/06 14:44:43 GMT FROM: Alexei Pozanenko at IKI, Moscow grb.alex@gmail.com

N. Pankov (HSE,IKI), A. Novichonok (Petrozavodsk State University, KIAM), S. Schmalz (KIAM), A. Volnova (IKI), A. Pozanenko (IKI) report on behalf of IKI GRB-FuN:

We observed the optical afterglow of GRB 251205A (Lanava et al., GCN 43005; Lipunov et al., GCN 43006; de Ugarte Postigo et al., GCN 43008; Malesani et al., GCN 43009; O'Neill et al., GCN 43010) using the Zeiss-1000 telescope at Koshka observatory (INASAN). The observations started/ended on 2025-12-06 (UT) 00:57:28/04:07:07. Preliminary photometry of the initial stacked image is the following

Date UT start t-T0 Exp. Filter OT Err. UL(3sigma) (mid, days) (s)

2025-12-06 00:57:28 0.05747 10*90 R 17.92 0.06 20.5

The photometry is based on nearby USNO-B1.0 stars and not corrected for Galaxy extinction.

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