GCN Circular 40945
Subject
EP250704a / GRB 250704B: VLT/FORS2 optical afterglow detection
Event
Date
2025-07-04T11:22:17Z (4 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
Daniele B. Malesani (DAWN/NBI and Radboud), Rob Eyles-Ferris (U. of Leicester), Gregory Corcoran (UCD), Massimiliano De Pasquale (University of Messina), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud), Antonio Martin-Carrillo (UCD), Aishwarya L. Thakur (INAF-IAPS), Dong Xu (NAOC) report on behalf of the Stargate collaboration:
We observed the field of EP250704a (Li et al., GCN 40941) / GRB 250704B (Wang et al., GCN 40940) using the ESO VLT UT1 (Antu) equipped with the FORS2 camera. Observations were carried out in the z band and were executed in rapid response mode (RRM).
The optical counterpart reported by Schneider et al. (GCN 40942) using COLIBRÍ/DDRAGO is clearly detected in our images. From a single 60-s exposure taken in the z band at a mean epoch of 2025 Jul 4.375 UT (42.8 min after the trigger), we measure
z = 20.12 +- 0.05 AB.
This is significantly brighter than the limit of the Pan-STARRS catalog, and well within the EP/FXT error circle. We thus concur with Schneider et al. (GCN 40942) that this is the likely afterglow of EP250704a.
We acknowledge expert support from the observing staff in Paranal, in particular Sam Kim, Robert Klement and Leonel Rivas.