TITLE: GCN CIRCULAR NUMBER: 39031 SUBJECT: EP250125a: REM optical counterpart detection DATE: 25/01/25 16:51:30 GMT FROM: Matteo Ferro at INAF-OAB matteo.ferro@inaf.it

M. Ferro, P. D’Avanzo, R. Brivio, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of the X-ray transient EP250125a, detected by Einstein Probe (Wu et al., GCN 39028) and Swift/XRT (Kennea et al., GCN 39029) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2025 January 25 at 02:44:32 UT (i.e. 493 seconds after the EP trigger), and lasting for about 3 hours. From preliminary photometry we detect the counterpart in the optical images at the position reported by Levan et al. (GCN 39027), with the following magnitudes:

r = 19.3 +- 0.3 (AB; calibrated against the Pan-STARRS catalogue) at a mid-time of t - t0 = 765 s after the trigger,

H > 15.8 (Vega; calibrated against the 2MASS catalogue)  at a mid-time of t - t0 = 641 s after the trigger.

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