The properties of GRB 120923A at a spectroscopic redshift of z=7.8
  Gamma-ray bursts (GRBs) are powerful probes of early stars and galaxies,
during and potentially even before the era of reionization. Although the number
of GRBs identified at z>6 remains small, they provide a unique window on
typical star-forming galaxies at that time, and thus are complementary to deep
field observations. We report the identification of the optical drop-out
afterglow of Swift GRB 120923A in near-infrared Gemini-North imaging, and
derive a redshift of z=7.84_{-0.12}^{+0.06} from VLT/X-shooter spectroscopy. At
this redshift the peak 15-150 keV luminosity of the burst was 3.2x10^52 erg/s,
and in fact the burst was close to the Swift/BAT detection threshold. The X-ray
and near-infrared afterglow were also faint, and in this sense it was a rather
typical long-duration GRB in terms of rest-frame luminosity. We present ground-
and space-based follow-up observations spanning from X-ray to radio, and find
that a standard external shock model with a constant-density circumburst
environment with density, n~4x10^-2 cm^-3 gives a good fit to the data. The
near-infrared light curve exhibits a sharp break at t~3.4 days in the observer
frame, which if interpreted as being due to a jet corresponds to an opening
angle of ~5 degrees. The beaming corrected gamma-ray energy is then
E_gamma~2x10^50 erg, while the beaming-corrected kinetic energy is lower,
E_K~10^49 erg, suggesting that GRB 120923A was a comparatively low kinetic
energy event. We discuss the implications of this event for our understanding
of the high-redshift population of GRBs and their identification.
