GCN Circular 42142
Subject
EP251003a: LCO and VST fading optical counterpart
Event
Date
2025-10-07T14:34:45Z (2 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), J. A. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (U. Leicester), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), F. E. Bauer (SSI and UTA), and J. A. Chacón (PUC) report on behalf of a larger collaboration:
We observed the location of EP251003a (Liu et al., GCN 42107) with the Sinistro instrument mounted on the 1-m telescopes at Cerro Tololo Inter-American Observatory, available via the
LCO network. Six 200 s exposures were obtained in the SDSS i filter starting at 2025-10-05 UT 07:16:29 (~35.1 hr post trigger).
Within the WXT and FXT error circles (Liu et al., GCN 42107), we identify a new source. Calibrated to nearby stars from the Pan-STARRS catalog, we measure a magnitude i = 21.6 +/- 0.2 (AB), not corrected for Galactic extinction.
No corresponding object is visible in the Legacy Survey down to an AB magnitude of i > 24.2.
We obtained an additional observation with the VLT Survey Telescope (VST). Six exposures of 300 s each were obtained in the SDSS i filter as well ten 200 s exposures in the SDSS z filter, starting at 2025-10-06 UT 06:47:12 (~58.6 hr post trigger). The counterpart identified in the LCO image is again detected, and we measure a magnitude of i = 22.87 +/- 0.07 and z = 23.23 +/- 0.14 (AB). Given the significant fading, the consistency with the X-ray localization, and the lack of a bright archival counterpart, we consider this source as the likely counterpart of EP251003a.
The most accurate coordinates of the counterpart as measured from the VST image are:
RA(J2000): 03:47:40.99
DEC(J2000): -10:55:55.0
We estimate an uncertainty of 0.2" in both RA and Dec.