TITLE: GCN CIRCULAR NUMBER: 30037 SUBJECT: GRB 210517A: VLT/X-shooter optical afterglow and redshift DATE: 21/05/17 13:07:00 GMT FROM: Daniele B Malesani at DTU Space N. R. Tanvir (Univ. Leicester), D. Xu (NAOC/CAS), L. Izzo (DARK/NBI), D. A. Kann (HETH/IAA-CSIC), D. B. Malesani (DTU space), J.-B. Vielfaure (APC), P. Jakobsson (Univ. Iceland), A. de Ugarte Postigo (HETH/IAA-CSIC and DARK/NBI), K. E. Heintz (Univ. Iceland and DAWN/NBI), V. D'Elia (ASI/SSDC), A. J. Levan (Radboud Univ.), S. D. Vergani (CNRS, Paris Obs.), S. Campana (INAF/Brera), G. Pugliese (Amsterdam), report on behalf of the Stargate collaboration: We observed the field of GRB 210517A (D'Elia et al., GCN 30032; Ursi et al., GCN 30036) 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 by 600 s each. The observation mid time was 2021 May 17.387 UT (3.82 hr after the GRB). In a 30 s image taken with the acquisition camera on May 17.364 UT, we detect a single object consistent with the UVOT-enhanced XRT position (Beardmore et al., GCN 30034), at J2000 coordinates: RA = 23:52:53.95 Dec = -39:06:08.3 We measure for this target an AB magnitude r = 22.3 +- 0.1 (calibrated against the Legacy Survey, but only very few, faint stars are present in the acquisition camera field of view). We note that at the same position, to within the uncertainties, there is a faint object visible in the Legacy Survey, with AB magnitude r ~ 23.7. We conclude that the above source is the optical afterglow of GRB 210517A, and the archival object is its host galaxy. In our spectra, continuum is detected over the whole observed range. A prominent emission line is detected around 4236 AA, which we identify as Ly alpha in emission. This interpretation is confirmed by the detection of both emission lines in the near-infrared arm (corresponding to [O II] and [O III]), and a plethora of metallic absorption features, among them Si II, Si IV, O I, C IV, C II, all at a common redshift z = 2.486. Consistent with the detection of bright Ly alpha in emission, the observed HI absorption column density is low. We note that at z = 2.486 the galaxy is a particularly bright GRB host, with an absolute AB magnitude M(1800 AA) ~ -21.5. We acknowledge excellent support from the ESO observing staff in Paranal, in particular Luca Sbordone, Francisco Caceres, and Boris Haeussler.