  We present the discovery of an extremely bright and extended lensed source
from the second Red Sequence Cluster Survey (RCS2). RCSGA 032727-132609 is
spectroscopically confirmed as a giant arc and counter-image of a background
galaxy at $z=1.701$, strongly-lensed by the foreground galaxy cluster RCS2
032727-132623 at $z=0.564$. The giant arc extends over $\sim 38$\,\arcsec and
has an integrated $g$-band magnitude of 19.15, making it $\sim 20$ times larger
and $\sim 4$ times brighter than the prototypical lensed galaxy MS1512-cB58.
This is the brightest distant lensed galaxy in the Universe known to date. Its
location in the `redshift desert' provides unique opportunities to connect
between the large samples of galaxies known at $z\sim3$ and $z\sim1$. We have
collected photometry in 9 bands, ranging from $u$ to $K_s$, which densely
sample the rest-frame UV and optical light, including the age-sensitive
4000\AA\ break. A lens model is constructed for the system, and results in a
robust total magnification of $2.04 \pm 0.16$ for the counter-image; we
estimate an average magnification of $17.2 \pm 1.4$ for the giant arc based on
the relative physical scales of the arc and counter-image. Fits of
single-component spectral energy distribution (SED) models to the photometry
result in a moderately young age, $t = 115 \pm 65$\,Myr, small amounts of dust,
$E(B-V) \le 0.035$, and an exponentially declining star formation history with
\textit{e}-folding time $\tau = 10-100$\,Myr. After correcting for the lensing
magnification, we find a stellar mass of
$\log(\mathrm{M}/\mathrm{M}_\odot)=10.0 \pm 0.1$. Allowing for episodic star
formation, an underlying old burst could contain up to twice the mass inferred
from single-component modeling. This stellar mass estimate is consistent with
the average stellar mass of a sample of `BM' galaxies ($1.4 < z < 2.0$) studied
by Reddy et al. (2006).
