TITLE: GCN CIRCULAR NUMBER: 38869 SUBJECT: GRB 250101A: Terskol K-800 optical observations DATE: 25/01/09 12:38:39 GMT FROM: Nicolai Pankov at HSE, IKI RAS colinsergesen@gmail.com

N. Pankov (HSE, IKI RAS), A. Pozanenko (IKI RAS), V. Agletdinov (KIAM), A. Moskvitin (SAO RAS) report on behalf of a larger collaboration:

We performed optical observations of the field of GRB 250101A (Page et. al, GCN 38752; Li et. al, GCN 38753; Mohan et. al, GCN 38754; Zhu et. al, GCN 38755; Budnev et. al, GCN 38756; Wu et. al, GCN 38758; Zhu et. al, GCN 38759; Goad et. al, GCN 38761; Moskvitin et. al, GCN 38762; Odeh et. al, GCN 38763; Hu et. al, GCN 38764; Moskvitin et. al, GCN 38765; D'Avino et. al, GCN 38768; Escudero-Coca et. al, GCN 38769; Leonini et. al, GCN 38771; Dichiara et. al, GCN 38773; Zhang et. al, GCN 38774; Siegel, GCN 38775; Li et. al, GCN 38776; Komesh et. al, GCN 38777; Ghosh et. al, GCN 38779; Moskvitin et al., GCN 38780; Krimm et. al, GCN 38782; Komesh et. al, GCN 38790; Méndez-Lapido et. al, GCN 38810; Gupta et. al, GCN 38818; Ferro et. al, GCN 38831; Lin et. al, GCN 38837) in the clear light with the 0.8-meter K-800 telescope of the Terskol observatory of INASAN. The observations began on 2025-01-01 18:09:54 UT, i.e. ~0.2 days since trigger. The preliminary photometry of the optical afterglow in the combined image of 160*30 sec is as follows:

Date UT start t-T0 Exp. Filter OT Err. UL(3sigma) (mid, days) (s) 2025-01-01 18:09:54 0.22713 160*30 Clear 20.03 0.18 21.5

The photometry is based on nearby stars of PS1 catalog (Lupton transformations) and not corrected for the Galactic extinction.

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