TITLE: GCN CIRCULAR NUMBER: 40086 SUBJECT: GRB 250404A / EP250404a: Mondy optical observations DATE: 25/04/06 11:27:55 GMT FROM: Alexei Pozanenko at IKI, Moscow grb.alex@gmail.com
N. Pankov (HSE), E. Klunko (ISTP), 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). 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) is early detected on 2025-04-04 and was observed between (UT) 14:50:52 and 17:09:01. We obtained an upper limit on 2025-04-05. Preliminary photometry of some stacked images is the following
Date UT start t-T0 Filter Exp. OT Err. UL(3sigma) (mid, days) (s)
2025-04-04 14:50:52 0.04244 R 40*90 16.53 0.02 22.4 2025-04-04 16:01:21 0.08999 R 28*120 17.96 0.03 22.2 2025-04-05 15:08:33 1.05611 R 42*90 n/d n/d 21.5
The photometry is based on nearby USNO-B1.0 stars.
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