A. Rossi (INAF-OAS), L. Izzo (HETH/IAA-CSIC), B. Milvang-Jensen
(DAWN/NBI), D. A. Perley (LJMU), A. de Ugarte Postigo (HETH/IAA-CSIC and
DARK/NBI), D. A. Kann (HETH/IAA-CSIC), A. J. Levan (U. Warwick), N. R.
Tanvir (U. Leicester), S. Covino (INAF/Brera), and D. B. Malesani
(DAWN/NBI and DARK/NBI) report on behalf of the Stargate collaboration:
We observed the optical counterpart of GRB 180728A (Starling et al., GCN
23046; Lipunov et al., GCN 23048) with the ESO VLT UT2 equipped with the
X-shooter spectrograph. One 600 s spectrum was taken during evening
twilight on 2018 July 28 beginning at 23:13 UT, plus two additional 600
s spectra on 2018 July 29 beginning at 05:10 UT. The spectra cover a
wavelength range from 3000-25000 AA.
A red continuum is detected across the spectral range. We detect
absorption features due to Mg II (3124,3132), Mg I (3187), and Ca II
(4395,4434) at a consistent redshift of z=0.117, which we propose as the
redshift of the GRB. Galactic ISM absorption features of Ca II and Na I
are also detected.
This GRB resembles the famous GRB 030329, but slightly less energetic
(E_iso = 2e+51 erg given the fluence value provided by Veres et al., GCN
23053). It is the closest energetic (E_iso > 1e+51 erg) GRB with a
measured redshift to date.
We note that although Galactic extinction in this direction is
significant (A_V = 0.763; Schlafly et al. 2011, ApJ 737:103) a campaign
to study the anticipated associated supernova should still be practical
with moderate-aperture telescopes at this redshift.
We acknowledge the ESO observing staff at Paranal, in particular Pascale
Hibon and R. Thomas.
Important Notice: the information in this email and any attachments is for the sole use of the intended recipient(s). If you are not an intended recipient, or a person responsible for delivering it to an intended recipient, you should delete it from your system immediately without disclosing its contents elsewhere and advise the sender by returning the email or by telephoning a number contained in the body of the email. No responsibility is accepted for loss or damage arising from viruses or changes made to this message after it was sent. The views contained in this email are those of the author and not necessarily those of Liverpool John Moores University.