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GCN Circular 23055

GRB 180728A: VLT/X-shooter redshift of a nearby energetic GRB
2018-07-29T18:23:26Z (6 years ago)
Daniel Perley at Liverpool JMU <>
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.

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