GCN 44209: EP260403a: LCO optical observations of the counterpart

J. Quirola-Vasquez (Radboud), A. Martin-Carrillo (UCD), J. N. D. van Dalen (Radboud), J. Chácon (PUC) and P. G. Jonker (Radboud) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP260403a (Guo et al., GCN 44204) with the LCO 1m telescope located at Siding Spring Observatory equipped with the SINISTRO instrument. We obtained 6 x 60 exposures in r-band, starting on 2026-04-03 at 12:19:37.245 UT (~1.84 hr post the EP trigger).

In our stacked images, we detect the optical counterpart reported by Antier et al., GCN 44206 and Pérez-Fournon et al. GCN 44207 with an AB magnitude of r = 19.35 +/- 0.05, calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction. Our observations confirm that the counterpart is fading rapidly.