GCN Circular 41760
Subject
EP250908c: VLT/FORS2 optical counterpart identification
Event
Date
2025-09-09T04:22:14Z (4 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), M. Garnichey (LUX-Paris Obs.), D. Xu (NAOC), L. Cotter (UCD), J. A. Chacon (PUC), L. Izzo (INAF/OACn and DARK/NBI), J. An (NAOC), report on behalf the Stargate collaboration:
We observed the field of the fast X-ray transient EP250908c (Zhang et al., GCN 41754) using the ESO VLT UT1 (Antu) equipped with the FORS2 instrument.
In a 150 s I-band observation with mid-time 2.76 hr after the high-energy trigger, we detect an uncatalogued object at J2000 coordinates:
RA = 00:32:52.18
Dec = -50:27:39.8
This position is consistent with the EP/FXT X-ray location (Zhang et al., GCN 41754) and nothing is visible at this location in archival imaging from the Legacy Survey. We thus conclude that this is the optical afterglow of EP250908c.
Using nearby calibrator stars from the SkyMapper catalog, we measure for the counterpart a magnitude i = 20.40 +/- 0.1 (AB), where most of the error is due to calibration scatter.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Marcela Espinoza, Julien Drevon, Marco Berton and Jonathan Smoker.