TITLE: GCN CIRCULAR NUMBER: 40090 SUBJECT: GRB 250404A / EP250404a: RC500 INASAN-Kislovodsk optical upper limit DATE: 25/04/06 13:05:06 GMT FROM: Alexei Pozanenko at IKI, Moscow grb.alex@gmail.com

N. Pankov (HSE), A.N. Tarasenkov (INASAN), A. Pozanenko (IKI) report on behalf of IKI GRB-FuN:

We observed GRB 250404A / EP250404a (Fermi GBM team, GCN 40050; Hu et al., GCN 40051; Frederiks et al., GCN 40087) with RC500 telescope of the INASAN-Kislovodsk observatory. The optical counterpart (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Zhu et al., GCN 40061; Konno et al., GCN 40063; Odeh et al., GCN 40064; He et al., GCN 40069; Pérez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al., GCN 40073; Taguchi et al., GCN 40074; Brivio et al., GCN 40075; Ruocco et al., GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., GCN 40083; Li et al., GCN 40084; Yin et al., GCN 40085; Pankov et al., GCN 40086; Chen et al., GCN 40088;) is not detected starting on 2025-04-04 (UT) 21:10:11. Preliminary upper limit of the stacked image is the following

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

2025-04-04 21:10:11 0.33362 R 70*120 n/d n/d 20.1

The photometry is based on nearby USNO-B1.0 stars.

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