GCN Circular 43928
Subject
EP260302a: Optical afterglow rebrightening
Event
Date
2026-03-05T23:34:56Z (6 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), L. Cotter (UCD), J. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud) report on behalf of a larger collaboration:
We observed the optical counterpart (Li et al., GCN 43898; Pérez-García et al., GCN 43903; Corcoran et al., GCN 43904) of the fast X-ray transient EP260302a (Zhang et al., GCN 43899; Zhang et al., GCN 43906) on two epochs. Our first observation was carried out with the Gemini South telescope located in Cerro Pachon (Chile); and the second with the LCO 1m telescope located in Sutherland (South Africa). The exposure times were 1x30 and 9x300 s, respectively, both in the SDSS r band, at mean epochs 2026-03-03 at 07:25:03 UT and 2026-03-05 at 19:05:14 UT (~7.8 and ~67.4 hr after trigger, respectively).
The counterpart is clearly detected on both epochs. We measure magnitudes r = 22.06 +- 0.12 (Gemini) and r = 21.07 +/- 0.06 (LCO). Both values are calibrated against nearby stars from the SkyMapper catalog, and are not corrected for Galactic extinction.
The observed fluxes imply a rebrightening by ~1 mag, which is uncommon for FXT and GRB afterglows around this epoch, and possibly indicates the emergence of an extra component (for another example, see e.g. van Dalen et al. 2025, doi:10.3847/2041-8213/adbc7e).
We encourage further multiwavelength follow up of this unusual source.