TITLE: GCN CIRCULAR NUMBER: 37583 SUBJECT: EP240919a / GRB 240919A: Mondy optical upper limit DATE: 24/09/21 10:30:10 GMT FROM: Alexei Pozanenko at IKI, Moscow grb.alex@gmail.com

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

We observed the field of EP240919a (Liang et al., GCN 37561) which is also confirmed as GRB 240919A (Liang et al., GCN 37563; Rodi et al., GCN 37572; Wang et al., GCN 37574) using the AZT-33IK telescope of Mondy Observatory in R-filter, starting on 2024-09-20 (UT) 14:33:23. We did not detect the optical counterpart. Preliminary photometry is the following:

Date UT Start t-T0 (days) Filter Exposure (s) OT Err UL (3σ) 2024-09-20 14:33:23 1.01091 R 30*120 n/d n/d 22.5

The magnitudes were calibrated using nearby USNO-B1.0 stars: RA DEC R2 22:16:57.1780584 -09:42:25.963092 15.91 22:17:14.9879832 -09:45:26.931528 17.1

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