GCN 44139: The EP-WXT trigger 01709259209 (EP260329b?): Las Cumbres discovery of the optical counterpart

2026-03-29T11:11:13.294Z | rev 0 | event: EP260329b

Wenxiong Li, Runduo Liang (NAOC), Iair Arcav, Ido Keinan (TAU), David Sand (U of Arizona)

We observed the position of EP-WXT trigger 01709259209 with a Las Cumbres 1m telescope at Siding Spring Observatory, Australia, 6 mins after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical w-band.
We find an uncataloged source at RA=181.3124, Dec=4.3516 within the EP-WXT error circle and measure the following preliminary photometry calibrated to the r band: 
Epoch 1: MJD 61128.446 Mag 18.7
Epoch 2: MJD 61128.450 Mag 17.1


The presented magnitudes are calibrated using Pan-STARRS sources in the field and are not corrected for Galactic extinction. We identify a faint source at this position in the Legacy Survey data, with 
r=24.08 mag. Additional follow-up observations are encouraged.