RCS2 J232727.6-020437: An Efficient Cosmic Telescope at $z=0.6986$
  We present a detailed gravitational lens model of the galaxy cluster RCS2
J232727.6-020437. Due to cosmological dimming of cluster members and ICL, its
high redshift ($z=0.6986$) makes it ideal for studying background galaxies.
Using new ACS and WFC3/IR HST data, we identify 16 multiple images. From
MOSFIRE follow up, we identify a strong emission line in the spectrum of one
multiple image, likely confirming the redshift of that system to $z=2.083$.
With a highly magnified ($\mu\gtrsim2$) source plane area of $\sim0.7$
arcmin$^2$ at $z=7$, RCS2 J232727.6-020437 has a lensing efficiency comparable
to the Hubble Frontier Fields clusters. We discover four highly magnified
$z\sim7$ candidate Lyman-break galaxies behind the cluster, one of which may be
multiply-imaged. Correcting for magnification, we find that all four galaxies
are fainter than $0.5 L_{\star}$. One candidate is detected at ${>10\sigma}$ in
both Spitzer/IRAC [3.6] and [4.5] channels. A spectroscopic follow-up with
MOSFIRE does not result in the detection of the Lyman-alpha emission line from
any of the four candidates. From the MOSFIRE spectra we place median upper
limits on the Lyman-alpha flux of $5-14 \times 10^{-19}\, \mathrm{erg \,\,
s^{-1} cm^{-2}}$ ($5\sigma$).
