GCN Circular 23315

J-B. Vielfaure (CNRS - GEPI/Observatorie de Paris), J. Japelj (API, U.
Amsterdam), D. B. Malesani (DAWN/NBI and DARK/NBI), V. D'Elia (SSDC), J. P.
U. Fynbo (DAWN/NBI), N. R. Tanvir (Univ. Leicester), S. D. Vergani (CNRS -
GEPI/Observatorie de Paris), A. Rossi (INAF-OAS), P. Vreeswijk (Radboud
Univ. Nijmegen), L. Izzo (HETH/IAA-CSIC), D. A. Kann (HETH/IAA-CSIC), B.
Milvang-Jensen (DAWN/NBI), D. Xu (NAOC) and K. E. Heintz (Univ. of
Iceland), report on behalf of the Stargate collaboration:

We observed the optical counterpart of GRB 181010A (Melandri et al. GCN
23309
Not found
 
 
; E. Gorbovskoy et al. 23310; R. Martone et al. 23311; E. Troja et al.
23312) with the ESO-VLT UT2 equipped with the X-shooter spectrograph. From
the acquisition image we measure a preliminary magnitude of 20.4 +/- 0.1
mag in the r-band, calibrated against local Pan-STARRS sources. Several
spectra with increasing exposure time were collected; in this analysis we
consider only one with 600s exposure obtained in a Rapid Response Mode at
06:18:24.9 UT (at the start of observation, 0.37 hr after the GRB).

A bright continuum is observed across the entire wavelength range covered
by X-shooter. Several absorption and emission features are detected, which
can be identified as due to H-alpha and [OIII]5007/4959 in emission at z =
1.39. At this same redshift we also detect AlII1854/1862, SiII1808, the
MgII doublet, and MgI in absorption. Fine-structure lines from FeII and
NiII are also detected at z = 1.39, which makes the redshift association
with the GRB secure. An intervening system at z = 0.67 with absorption
features due to the MgII doublet and MgI is also identified in the
spectrum.

We acknowledge the ESO observing staff at Paranal, in particular Jorge
Lillo-Box.