TITLE: GCN CIRCULAR NUMBER: 39585 SUBJECT: EP250304a: VLT/X-shooter redshift z = 0.200 DATE: 25/03/04 09:54:03 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl

A. Saccardi (CEA/Irfu), Z. P. Zhu (NAOC), B. Schneider (LAM), D. Xu (NAOC), L. Izzo (INAF/OAC and DARK/NBI), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (U. Leicester), S. D. Vergani (CNRS - Paris Observatory/LUX) report on behalf of the Stargate collaboration:

We observed the optical counterpart (Liu et al., GCN 39583; see also Page et al., GCN 39584) of the fast X-ray transient EP250304a (Chen et al., GCN 39580) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.

In a 30-s r-band image taken with the acquisition camera (mid-time 2025 Mar 4.302 UT, or 5.7 hr after trigger), we detect the optical counterpart with a magnitude r = 20.97 +/- 0.03 (AB), calibrated against nearby stars from the SkyMapper catalog (Wolf et al. 2018, doi:10.4225/41/593620ad5b574).

Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. The observation mid time was 2025 Mar 4.3248 UT (6.254 hr after the GRB).

In a preliminary reduction of the spectra, we detect continuum over the entire covered wavelength range. We detect emission lines in the VIS arm, which we identify as Halpha and [O III] 5007 AA at the common redshift of z = 0.200. We also identify the Mg II doublet (2796,2804) in absorption at a consistent redshift.

The spectrum continues to rise towards the blue end down to the cut-off of the UVB arm (~3100 AA). This spectral shape is qualitatively similar to what was seen in EP250108a / AT2025kg (e.g., Zhu et al., GCN 38908). We encourage further follow-up of this potentially interesting target.

We acknowledge expert support from the ESO staff in Paranal, in particular Diego Parraguez.

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