TITLE:   GCN CIRCULAR
NUMBER:  14273
SUBJECT: GRB 111123A: Keck-I host detection and VLT/X-shooter redshift
DATE:    13/03/08 10:59:57 GMT
FROM:    Dong Xu at DARK/NBI  <dong.dark@gmail.com>

D. Xu, D. Malesani, T. Kruehler, J. P. U. Fynbo (DARK/NBI), D. A.
Perley (Caltech), P. Goldoni (APC/Univ. Paris 7 and SAp/CEA), L. Kaper
(U. Amsterdam), A. de Ugarte Postigo (IAA-CSIC and DARK/NBI), N. R.
Tanvir (U. Leicester) report on behalf of the X-shooter GTO GRB
collaboration:

We observed the field of GRB 111123A (Stamatikos et al., GCN 12587)
using the Keck-I telescope equipped with the LRIS instrument.
Observations were carried out on 2013 February 10 (~444 days after the
burst), simultaneously in the g and I bands, for a total exposure time
of 750 and 720 s, respectively.

An extended source with g=25.83 (AB) and I = 23.55 (Vega) is detected
at the position of the optical and NIR afterglow (Xu et al., GCN
12589; Fugazza et al., GCN 12593), where the positional error radius
of the optical afterglow has been reduced from ~1.0" to ~0.3" through
a refined analysis. We thus consider the source to be the host galaxy
of GRB 111123A.

A spectrum of this source was taken on 2013 March 07 with the ESO VLT
equipped with the X-shooter spectrograph, featuring NIR/VIS/UVB three
arms and covering the wavelength range 3000-25000 AA. The exposure
time was 4x600 s. In the NIR arm, we detect four emission lines,
interpreted as [O III] (5007), [NeIII] (3869), [O II] (3727), and
Hbeta (a marginal detection), all at a common redshift z = 3.1516. In
the UVB arm, the host continuum is detected down to ~5100 AA, thus
corresponds to the onset of the Lyman alpha forest at the proposed
redshift.

We acknowledge excellent support from the observing staff at Mauna Kea
and Paranal, in particular Emanuela Pompei, Claudio Melo, and Andres
Pino.