GCN Circular 14225
Subject
GRB 120118B: host observations and redshift determination
Date
2013-02-17T01:46:52Z (11 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@dark-cosmology.dk>
Daniele Malesani, Thomas Kruehler (DARK/NBI), Daniel Perley (Caltech),
Johan P. U. Fynbo, Dong Xu, Bo Milvang-Jensen (DARK/NBI), Paolo Goldoni
(APC, CEA/Irfu), Steve Schulze (PUC and MCSS), report on behalf of the
X-shooter GTO GRB collaboration:
We observed the field of GRB 120118B (Littlejohns et al., GCN 12852)
with the Keck I telescope located on Mauna Kea. Observations were
carried out on 2013 Feb 10 (389 days after the GRB), using the LRIS
instrument, simultaneously in the g and I bands, for a total exposure
time of 750 and 720 s, respectively. Consistent with the latest XRT
position (Osborne et al. GCN 12857; see also
http://www.swift.ac.uk/xrt_positions/), we detect a bright, slightly
extended object (I = 23.8, Vega), which we consider to be the GRB host
galaxy. Its coordinates are (J2000):
RA = 08:19:29.047
Dec = -07:11:05.14
This position is consistent with that of the tentative near-infrared
counterpart reported by D'Avanzo & Palazzi (GCN 12870).
A spectrum of this source was taken on 2013 Feb 13 with the ESO VLT
equipped with the X-shooter spectrograph, covering the wavelength range
3000-20500 AA. The seeing was 0.6". In the NIR arm, we detect several
emission lines, interpreted as [O III] (5007), [Ne III] (3869), and
hints for [O III] (4959) and [O II] (3727) at a common redshift z =
2.943. The weakness/lack of other prominent lines usually seen in GRB
host spectra is readily explained by their location in non-favorable
parts of the spectrum. In the UVB arm, the host continuum is detected
down to ~4750 AA, which corresponds to the onset of the Lyman alpha
forest at z = 2.943.
At z = 2.94 the host is a fairly luminous galaxy at 2000 � rest-frame,
roughly 0.5 mag brighter than L* at that redshift (e.g. Gabasch et al.
2004, A&A, 421, 41). The properties of this object are akin to those of
Lyman-break galaxies.
We acknowledge excellent support from the observing staff at Mauna Kea
and Paranal, in particular Cedric Ledoux and Andrea Mehner.