TITLE: GCN CIRCULAR NUMBER: 39487 SUBJECT: EP250226a / GRB 250226A: VLT/X-shooter redshift of z = 3.315 DATE: 25/02/26 10:21:06 GMT FROM: Zipei Zhu at NAOC zpzhu@nao.cas.cn

Z. P. Zhu (NAOC), A. Saccardi (CEA/Irfu), B. Schneider (LAM), L. Izzo (INAF-OACn and DARK/NBI), J. T. Palmerio (CEA/Irfu), D. Xu (NAOC), N. R. Tanvir (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud Univ.), G. Pugliese (API-UvA), A. J. Levan (Radboud and Warwick), S. D. Vergani (CNRS, Obs. Paris/LUX) report on behalf of the Stargate collaboration:

We observed the optical afterglow (An et al., GCN 39486) of EP250226a / GRB 250226A (Fermi GBM team, GCN 39479; Jiang et al., GCN 39482) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 2x600 s. The observation mid-time was 2025 Feb. 26 at 08:43:57 (2.15 hr after the GBM trigger).

In a 15 s image taken with the acquisition camera on Feb. 26 at 08:27:06 UT, we detect the optical afterglow, for which we measure a magnitude r =19.17 +/- 0.03 AB (calibrated against nearby stars from the Pan-STARRS catalogue).

In a preliminary reduction of the spectra, we clearly detect a continuum down to 4000 AA. From the detection of a broad Lya absorption at ~5230 AA and multiple narrow absorption features, including O I, O I*, Si II, Si II*, C II, C II*, C IV, Si IV, Al II, Fe II, Fe II*, Al III, Ni II, Ni II*, Zn II, Cr II, Mg II, Mg I, Ca II we infer the redshift of the GRB to be z = 3.315.

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

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