Johan P. U. Fynbo (DAWN/NBI and DAWN/DTU), Antonio de Ugarte Postigo
(HETH/IAA-CSIC and DARK/NBI), Valerio D'Elia (ASI-SSDC), Nial R. Tanvir
(Univ. Leicester), Giovanna Pugliese (API/Univ. Amsterdam), D. Alexander
Kann (HETH/IAA-CSIC), D. Malesani (DAWN/NBI, DAWN/DTU, and DARK/NBI),
and Kasper E. Heintz (Univ. Iceland), report on behalf of the Stargate
We observed the optical afterglow of the bright Swift and Fermi GRB
181020A (Moss et al., GCN 23349; Axelsson, GCN 23350; Veres, GCN 23352)
with the ESO VLT UT2 equipped with the X-shooter instrument.
Spectroscopy covering the wavelength range 3000-21000 AA was carried out
starting on 2018 Oct 21.030 UT (5.7 hr after the GRB), for a total of
4x600 s exposure time.
In a 5-s image taken with the acquisition camera on 2018 Oct 10.023 UT,
we measure for the afterglow R ~ 17.3 (Vega), assuming R = 18.34 for the
USNO star at RA = 00:55:51.53, Dec = -47:23:14.5.
The spectrum has a very high S/N. A wide trough is easily visible around
4780 AA, which we interpret as a DLA in the GRB host system. From
detection of a plethora of absorption lines, including Al II, Al III, S
II, O I, Si II, Si IV, C IV, Zn II, Cr II, Ni II, Fe II, Mn II, O I*, Si
II*, Ni II*, Fe II*, we measure the redshift to be z = 2.938. The
velocity structure of the lines spans the redshift range 2.9360-2.9407
(2.938 corresponding to the strongest component). The detection of
fine-structure lines makes the redshift association with the GRB host
system secure. Intervening absorbers are also detected at z = 1.788 (Mg
II), 2.172 (Mg II), and 2.822 (C IV).
We acknowledge the ESO observing staff in Paranal for conducting these
observations, in particular Elizabeth Bartlett, Stephane Brillant,
Marcelo Lopez, and Thomas Rivinius.