TITLE: GCN CIRCULAR NUMBER: 38704 SUBJECT: GRB 241228B: VLT/X-shooter redshift of z = 2.674 DATE: 24/12/29 10:08:10 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

J. An (NAOC), B. Schneider (LAM), A. Saccardi (CEA/Irfu), J. T. Palmerio (CEA/Irfu), D. Xu (NAOC), A. Kumar (RHUL/Warwick), A. J. Levan (Radboud Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:

We observed the field of the GRB 241228B triggered by Fermi (Fermi GBM team, GCN 38682) 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 with mid-time 06:35:08 UT on 2024 December 29 (~1.1 days after the trigger).

In the image taken with the acquisition camera, we detect the proposed optical afterglow (Kumar et al., GCN 38684 and GCN 38691; An et al. GCN 38687; Ortega-Casas et al., GCN 38692; Ghosh et al., GCN 38702), for which we measure an AB magnitude r = 20.7 +/- 0.1, calibrated against nearby stars from the Legacy Survey DR10 catalog.

In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range. From the detection of a broad Lya absorption at ~4470 AA and multiple absorption features, which interpreted as being due to NV, SII, SiII, OI, CII, SiIV, CIV, FeII, AlII, AlIII, NiII, CrII, ZnII, MgII, MgI, SiII*, OI*, CII*, FeII*, NiII*, we infer a common redshift of z = 2.674. The detection of fine-structure lines confirms the afterglow nature of the source, and we conclude this is the redshift of the burst. A strong Lyman alpha emission line is detected at a consistent redshift, which we interpret as being due to the GRB host galaxy.

We also note the presence of additional absorption features likely due to multiple intervening systems at z = 2.000 and z = 1.824.

We acknowledge expert support from the ESO staff in Paranal.

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