D. B. Malesani (DTU Space), A. de Ugarte Postigo (HETH/IAA-CSIC,
DARK/NBI), D. A. Kann (HETH/IAA-CSIC), V. D'Elia (SSDC), J. P. U. Fynbo
(DAWN/NBI), and G. Pugliese (Univ. Amsterdam) report on behalf of the
We observed the localization of the Swift GRB 191011A (Laha et al., GCN
25988) with X-shooter on the VLT in RRM mode. The optical afterglow
(Laha et al., GCN 25988, Lipunov et al., GCN 25989) is well-detected,
and we measure a magnitude of r' = 19.18 +/- 0.06 mag (AB) from the 15 s
acquisition image obtained starting at 2019-10-11T04:59:12.540, 23.28
min after the trigger, measured against PanSTARRS field stars.
In the first spectrum (integration time 175 s) we detect Ly-a, SII, OI,
SiII, CII, SiIV, CIV, FeII, CI, AlII, AlIII, MnII, MgII, MgI, and FeII*,
all at a common redshift z=1.722. We consider this the redshift of GRB
191011A. We furthermore report the presence of an intervening MgII
system at z=1.2072.
We acknowledge excellent support from the ESO observing staff in
Paranal, in particular Chiara Mazzucchelli, Rodrigo Palominos, and
NB: We note the detection of a Fermi GRB (The Fermi GBM team, GCN 25987)
at essentially the same time and with similar light curve structure,
however, this GRB is localized to the northern hemisphere.