TITLE: GCN CIRCULAR NUMBER: 37532 SUBJECT: GRB 240916A (AT 2024vlp / GOTO24fzn): VLT/X-shooter redshift DATE: 24/09/17 09:18:25 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl

D. Pieterse (Radboud Univ.), B. Schneider (MIT), A. Saccardi (GEPI/Obs. de Paris), B. P. Gompertz (U. Birmingham), J. P. U. Fynbo (DAWN/NBI), A. de Ugarte Postigo (LAM/OCA, CNRS), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), N. R. Tanvir (Univ. Leicester), D. Xu (NAOC) report on behalf of the Stargate collaboration:

We observed AT 2024vlp (GOTO24fzn), the likely afterglow (Gompertz et al., GCN 37522; Jiang et al., GCN 37526; Beardmore et al., GCN 37531) of GRB 240916A (Fermi GBM team, GCN 37518; Pawar, GCN 37519) using the X-shooter spectrograph mounted on the ESO VLT UT3 (Melipal). The observation was performed on 2024 September 16 (23.1 hr after the GRB), and consisted of 4 exposures of 600 s each covering the wavelength range 3000-21000 AA.

In images taken with the acquisition camera, the optical afterglow is well detected. In the r band, we measure an AB magnitude of r ~ 19.4, calibrated against three nearby stars from the Pan-STARRS catalog. This magnitude is significantly fainter than that reported by Jiang et al. (GCN 37526), confirming the fading nature of this object and thus the identification of this source as the afterglow of GRB 240916A.

In a preliminary analysis of the spectra, we clearly detect continuum over the entire wavelength range. A trough is visible around 4350 AA, which we identify as due to HI Lyman-alpha absorption. In addition, from detection of a plethora of absorption features, which we interpret as being due to O I, O I*, Si II, Si II*, C II, C II*, Si IV, C IV, Fe II, Fe II*, Cr II, Zn II, Ni II, Ni II*, Mn II, Mg II, we infer a common redshift of z = 2.610, which we conclude is the redshift of GRB 240916A. There is a tentative detection of H-beta emission at z=2.610 from the underlying host galaxy. We also note the presence of an additional intervening absorption system at z = 2.214.

We acknowledge expert support from the ESO staff in Paranal, in particular Claudia Cid, Thomas Rivinius and Zahed Wahhaj.

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