[vsnet-grb-info 27934] GRB 210517A: VLT/X-shooter optical afterglow and redshift

[GCN Circulars]

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  <malesani at space.dtu.dk>

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.