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.