TITLE: GCN CIRCULAR NUMBER: 39020 SUBJECT: EP250108a: AbAO, CMO and Terskol optical upper limits DATE: 25/01/24 06:26:37 GMT FROM: Nicolai Pankov at HSE, IKI RAS colinsergesen@gmail.com

N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Tarasenkov (INASAN), R. Ya. Inasaridze (AbAO), I. Sokolov (INASAN) report on behalf of the IKI-GRB-FuN collaboration:

We performed optical observations of the field of EP250108a (Li et. al, GCN 38861; V.Lipunov et. al, GCN 38865; Eyles-Ferris, GCN 38878; Zhu et. al, GCN 38885; Li et. al, GCN 38888; et. al, GCN 38891; Malesani et. al, GCN 38902; Kumar et. al, GCN 38907; Zhu et. al, GCN 38908; Levan et. al, GCN 38909; Izzo, GCN 38912; Zou et. al, GCN 38914; Moskvitin et. al, GCN 38925; Zhu et. al, GCN 38937; Carotenuto et. al, GCN 38958; Schroeder et. al, GCN 38970; Song et. al, GCN 38972; Eyles-Ferris et. al, GCN 38983; Xu et. al, GCN 38984; Levan et. al, GCN 38987; An et. al, GCN 38998; Ror et. al, GCN 39002) in the R filter with AS-32 of the Abastumani Observatory (AbAO), Zeiss-2000 and RC-500 telescopes of the Terskol (INASAN) Observatory, and RC-500 telescope of the Caucasian Mountain Observatory (CMO). The observations started on 2025-01-11 at 15:03 UT, i.e. ~3.11 days since trigger. We do not detect the optical counterpart in the stacked images. The preliminary photometry is as follows:

Date UT start t-T0 Exp. Filter UL(3sigma) Site (mid, days) (s) 2025-01-11 15:03:26 3.19199 247*60 R 19.6 CMO/RC-500 2025-01-11 16:06:14 3.17622 76*60 R 19.8 AbAO/AS-32 2025-01-11 16:25:13 3.19184 166*30 R 20.6 Terskol/Zeiss-2000 2025-01-20 15:16:48 12.27071 100*120 R 19.8 CMO/RC-500 2025-01-21 15:39:53 13.15306 31*120 R 18.2 Terskol/RC-500

The photometry has been calibrated using nearby stars from the USNO-B1.0 catalog (R2 magnitudes). No correction has been made for the Galactic extinction.

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