GCN 44144: EP260329b: LCO optical observations of the counterpart

2026-03-29T13:42:38.324Z | rev 0 | event: EP260329b
R. A. J. Eyles-Ferris (Leicester), A. van Hoof (Radboud), G. Corcoran (UCD), J. Chácon (PUC), A. Martin-Carrillo (UCD), J. Quirola-Vasquez (Radboud),  A. J. Levan (Radboud), F. E. Bauer (SSI and UTA) and P. G. Jonker (Radboud) report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP260329b (Song et al., GCN 44142) with LCO 1m telescopes located at the Siding Spring Observatory (Australia) equipped with the SINISTRO instrument. We obtained 6 x 300 exposures in each of the r and z filters, starting from 2026-03-29 12:03:17 UT (t_mid = 1.74 hr and 1.78 hr post trigger respectively).

We clearly detect the optical counterpart reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140) in our stacked images and measure the following AB magnitudes calibrated to Pan-STARRS and not corrected for Galactic extinction:

r = 19.84 +/- 0.05
z = 19.38 +/- 0.05

Our measurements are >1 magnitude fainter than those reported by Li et al. (GCN 44139) and Zhu et al. (GCN 44140) indicating the counterpart is fading rapidly.