TITLE: GCN CIRCULAR NUMBER: 40063 SUBJECT: EP250404a: LAST detection of optical counterpart DATE: 25/04/04 18:17:33 GMT FROM: Ruslan Konno at Weizmann Institute of Science ruslan.konno@desy.de

R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), P. Chen (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), S. Spitzer (WIS), and K. Rybicki (WIS) on behalf of the LAST Collaboration

We report observations of the X-ray transient EP250404a (Hu et al., GCN 40051), detected by Einstein Probe, with the Large Array Survey Telescope (LAST; Ofek et al. 2023 PASP 135, 5001; Ben-Ami et al. 2023 PASP 135, 5002).

We observe the field of EP250404a using 4 parallel telescopes, each with a FoV of 7.4 deg^2 and no filter (clear - similar to the GAIA Bp band) over several epochs. In each epoch, we coadd 20 images with each of 20s exposure, and the limiting magnitude of a single coadd is around 20.5 (AB mag). We clearly detect the optical counterpart reported by Jiang et al. (GCN 40052), Perez-Garcia et al. (GCN 40053), and Du et al. (GCN 40058). The earliest detection with a 20x20s exposure image is confirmed at 2025-04-04 17:01:04 UTC (T-T0=2.68h) at an AB magnitude of 19.31 +/- 0.07. LAST is scheduled to continue observations of the source, and additional analysis will follow.

LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).

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