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