TITLE: GCN CIRCULAR NUMBER: 39772 SUBJECT: GRB 250317B: Mondy AZT-33IK Optical Observations DATE: 25/03/18 15:19:56 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) in the R filter with the 1.5-meter AZT-33IK telescope of the Sayan Solar Observatory (Mondy). The observations started on (UT) 2025-03-17 15:41:49, i.e. about 0.58 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; Zhu et. al, GCN 39770) is clearly visible in the stacked image. The preliminary photometry is as follows:
Date UT start t-T0 Exp. Filter OT Err UL(3sigma) (mid, days) (s) 2025-03-17 15:41:49 0.58173 28*120 R 20.94 0.06 23.3
The photometry is based on nearby stars from the USNO-B1.0 catalog (R2 magnitudes) and has not been corrected for the Galactic extinction. Using a simple power law fit to the observed data in the r and R filters, we estimate the light curve decay index to be about -1.5.
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