[vsnet-grb-info 12881] GRB 120118B: host observations and redshift determination

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14225
SUBJECT: GRB 120118B: host observations and redshift determination
DATE:    13/02/17 01:46:52 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at 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 Å 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.