TITLE: GCN CIRCULAR NUMBER: 39790 SUBJECT: GRB 250317B: Mondy AZT-33IK Continued Optical Observations DATE: 25/03/20 13:21:09 GMT FROM: Nicolai Pankov at HSE, IKI RAS colinsergesen@gmail.com

N. Pankov (HSE, IKI), E. Klunko (ISTP), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of GRB 250317B (Zhao et. al, GCN 39753; Götz et. al, GCN 39756; Melandri et. al, GCN 39763) at the redshift of z=3.44 (Thoene et. al, GCN 39769) in the R filter with the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations started on 2025-03-18 at 18:45:47 (UT), i.e. about 1.72 days since trigger. The optical afterglow (Watson et. al, GCN 39754; Pérez-Fournon et. al, GCN 39757; Ghosh et. al, GCN 39759; Ferro et. al, GCN 39764; Schneider et. al, GCN 39767; Thoene et. al, GCN 39769; Zhu et. al, GCN 39770; Pankov et. al, GCN 39772; Moretti & Pavoni, GCN 39774; Schneider et. al, GCN 39777; Atteia et. al, GCN 39784) is not detected. The preliminary photometry is as follows:

Date UT start t-T0 Exp. Filter OT Err UL(3sigma) (mid, days) (s) 2025-03-18 18:45:47 1.72130 45*120 R n/d n/d 22.9 2025-03-19 18:02:14 2.70077 59*120 R n/d n/d 23.3

Ref. stars from SDSS-DR12 RA Dec R R_err 211.6590 +40.0703 16.451 0.029 211.5997 +40.0619 18.859 0.055

The photometry is based on nearby stars (see the list above) from the SDSS-DR12 catalog (R magnitudes were obtained using Lupton 2005 transformations) and has not been corrected for the Galactic extinction.

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