TITLE: GCN CIRCULAR NUMBER: 37293 SUBJECT: GRB 240825A: VLT/X-shooter redshift DATE: 24/08/26 08:44:45 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group antonio.martin-carrillo@ucd.ie

A. Martin-Carrillo (UCD), B. Schneider (MIT), G. Pugliese (API-UvA), L. Izzo (INAF-OACn & DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Saccardi (GEPI/Obs. de Paris), T. Laskar (Utah), J. F. Agui Fernandez (CAHA) and S. D. Vergani (GEPI/Obs. de Paris) report on behalf of the Stargate collaboration:

We observed the optical counterpart of the Swift/Fermi GRB 240825A (Gupta et al., GCN 37274; Fermi GBM team, GCN 37273; Di Lalla et al., GCN 37288) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 1200 s each. The observation was carried out under good seeing (1”) with mid-time 03:03:55 UT on 2024 August 26 (~11.2 hours after the Swift trigger).

In a 60 s image taken with the acquisition camera, we clearly detect the optical afterglow (Gupta et al., GCN 37274; Jiang et al. GCN 37275; Dutton et al. GCN 37276; Odeh et al. GCN 37277; Zhang et al. GCN 37278; Li et al. GCN 37280; Izzo & Malesani, GCN 37287; Lipunov et al. GCN 37289; Leonini et al. GCN 37291; Wu et al. GCN 37292), for which we measure an AB magnitude r = 20.8 (calibrated against nearby stars from the Pan-STARRS catalogue).

In a preliminary reduction of the spectra, we clearly detect the continuum over the entire wavelength range. From detection of multiple absorption features, which we interpret as due to Fe II, Fe II*, Mn II, Mg II, Mg I, Ca II and Na I, we infer a common redshift of z = 0.659. We also detect bright emission lines ([O II] and [O III] doublets, Halpha and Hbeta) at a consistent redshift, which we interpret as being due to the GRB host galaxy (Izzo & Malesani, GCN 37287). We conclude that GRB 240825A is at z = 0.659.

We acknowledge expert support from the ESO staff in Paranal, in particular Matias Jones.

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