A highly elongated prominent lens at z=0.87: first strong lensing
  analysis of El Gordo
  We present the first strong-lensing (SL) analysis of the galaxy cluster
ACT-CL J0102-4915 (\emph{El Gordo}), in recent \emph{HST}/ACS images, revealing
a prominent strong lens at a redshift of $z=0.87$. This finding adds to the
already-established unique properties of \emph{El Gordo}: it is the most
massive, hot, X-ray luminous, and bright Sunyaev-Zeldovich effect cluster at
$z\gtrsim0.6$, and the only `bullet'-like merging cluster known at these
redshifts. The lens consists of two merging massive clumps, where for a source
redshift of $z_{s}\sim2$ each clump exhibits only a small, separate critical
area, with a total area of $0.69\pm0.11\sq\arcmin$ over the two clumps. For a
higher source redshift, $z_{s}\sim4$, the critical curves of the two clumps
merge together into one bigger and very elongated lens (axis ratio
$\simeq5.5$), enclosing an effective area of $1.44\pm0.22\sq\arcmin$. The
critical curves continue expanding with increasing redshift so that for
high-redshift sources ($z_{s}\gtrsim9$) they enclose an area of
$\sim1.91\pm0.30\sq\arcmin$ (effective $\theta_{e}\simeq46.8\pm3.7\arcsec$) and
a mass of $6.09\pm1.04\times10^{14}M_{\odot}$. According to our model, the area
of high magnification ($\mu>10$) for such high redshift sources is
$\simeq1.2\sq\arcmin$, and the area with $\mu>5$ is $\simeq2.3\sq\arcmin$,
making \emph{El Gordo} a compelling target for studying the high-redshift
Universe. We obtain a strong lower limit on the total mass of \emph{El Gordo},
$\gtrsim1.7\times10^{15}M_{\odot}$ from the SL regime alone, suggesting a total
mass of, roughly, $M_{200}\sim2.3\times10^{15}M_{\odot}$. Our results should be
revisited when additional spectroscopic and \emph{HST} imaging data are
available.
