GCN Circular 41790
Subject
EP250911a: LCO optical counterpart discovery
Event
Date
2025-09-11T10:23:38Z (2 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
J. A. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), A. Martin-Carrillo (UCD), L. Cotter (UCD), P. G. Jonker (Radboud), F. E. Bauer (SSI and UTA), and J. Chacón (PUC) report on behalf of a larger collaboration:
We observed the location of EP250911a (Liang et al., GCN 41788) with the Sinistro instrument mounted on the 1-m telescopes at McDonald Observatory, available via the LCO network. 6 x 200 s exposures were obtained in the SDSS r filter at a start time of 2025-09-11 UT 09:35:35 (~37.1 min post trigger).
Within the WXT and FXT error circles (Liang et al., GCN 41788) we locate a new source in our stacked r-band image, not visible in the archival Legacy Survey images. Its coordinates (0.5" error) are:
RA(J2000): 23:50:04.83
DEC(J2000): +29:17:59.6
Calibrated to nearby stars from the Pan-STARRS catalog, we measure a magnitude r = 20.04 +/- 0.07 (AB), not corrected for Galactic extinction.
Given the non-detection in the deeper archival images and the consistency with the FXT error circle, we suggest that this source is the optical counterpart of EP250911a.