TITLE: GCN CIRCULAR NUMBER: 40049 SUBJECT: GRB 250402A: Mondy optical observations DATE: 25/04/04 14:24:06 GMT FROM: Alexei Pozanenko at IKI, Moscow grb.alex@gmail.com

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

We observed GRB 250402A (Chen et al. GCN 40014) with the AZT-33IK telescope starting on 2025-04-03 (UT) 18:11:28. We found the optical counterpart (Xin et al., GCN 40015; Pérez-Fournon et al., GCN 40020; Schneider et al., GCN 40022; Magnani et al., GCN 40023) at the position reported by Xin et al., GCN 40015.

Preliminary photometry of the optical counterpart is the following

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

2025-04-03 18:11:28 0.81567 R 29*120 21.9 0.2 22.3

The photometry is based on the nearby PS1 stars (R mag by Lupton transformations).

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