A spectroscopically confirmed z=1.327 galaxy-scale deflector magnifying
  a z~8 Lyman-Break galaxy in the Brightest of Reionizing Galaxies survey
  We present a detailed analysis of an individual case of gravitational lensing
of a $z\sim8$ Lyman-Break galaxy (LBG) in a blank field, identified in Hubble
Space Telescope imaging obtained as part of the Brightest of Reionizing
Galaxies survey. To investigate the close proximity of the bright
($m_{AB}=25.8$) $Y_{098}$-dropout to a small group of foreground galaxies, we
obtained deep spectroscopy of the dropout and two foreground galaxies using
VLT/X-Shooter. We detect H-$\alpha$, H-$\beta$, [OIII] and [OII] emission in
the brightest two foreground galaxies (unresolved at the natural seeing of
$0.8$ arcsec), placing the pair at $z=1.327$. We can rule out emission lines
contributing all of the observed broadband flux in $H_{160}$ band at
$70\sigma$, allowing us to exclude the $z\sim8$ candidate as a low redshift
interloper with broadband photometry dominated by strong emission lines. The
foreground galaxy pair lies at the peak of the luminosity, redshift and
separation distributions for deflectors of strongly lensed $z\sim8$ objects,
and we make a marginal detection of a demagnified secondary image in the
deepest ($J_{125}$) filter. We show that the configuration can be accurately
modelled by a singular isothermal ellipsoidal deflector and a S\'{e}rsic source
magnified by a factor of $\mu=4.3\pm0.2$. The reconstructed source in the
best-fitting model is consistent with luminosities and morphologies of $z\sim8$
LBGs in the literature. The lens model yields a group mass of
$9.62\pm0.31\times10^{11} M_{\odot}$ and a stellar mass-to-light ratio for the
brightest deflector galaxy of $M_{\star}/L_{B}=2.3^{+0.8}_{-0.6}
M_{\odot}/L_{\odot}$ within its effective radius. The foreground galaxies'
redshifts would make this one of the few strong lensing deflectors discovered
at $z>1$.
