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 intra-cluster light, its high redshift (z = 0.6986) makes it ideal for studying highredshift background galaxies. Using new Advanced Camera for Surveys and WFC3/IR Hubble Space Telescope data, we identify 16 new multiple images belonging to 6 distinct source galaxies. From Multi-object Spectrometer for Infrared Exploration (MOSFIRE) follow-up, we identify a strong emission line in the spectrum of one multiple image, measuring the redshift of that system to z = 2.083. With a highly magnified (mu greater than or similar to 3) source plane area of similar to 0.9 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 similar to 7 candidate Lyman-break galaxies behind the cluster. Correcting for magnification, we find that all four galaxies are fainter than 0.5L(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 Ly alpha emission line from any of the four candidates. From the MOSFIRE spectra, we place median upper limits on the Ly alpha flux of (3-11) x 10(-19) erg s(-1) cm(-2) (5 sigma).