CLASH: New Multiple-Images Constraining the Inner Mass Profile of MACS
  J1206.2-0847
  We present a strong-lensing analysis of the galaxy cluster MACS J1206.2-0847
($z$=0.44) using UV, Optical, and IR, HST/ACS/WFC3 data taken as part of the
CLASH multi-cycle treasury program, with VLT/VIMOS spectroscopy for some of the
multiply-lensed arcs. The CLASH observations, combined with our mass-model,
allow us to identify 47 new multiply-lensed images of 12 distant sources. These
images, along with the previously known arc, span the redshift range $1\la
z\la5.5$, and thus enable us to derive a detailed mass distribution and to
accurately constrain, for the first time, the inner mass-profile of this
cluster. We find an inner profile slope of $d\log \Sigma/d\log \theta\simeq
-0.55\pm 0.1$ (in the range [1\arcsec, 53\arcsec], or $5\la r \la300$ kpc), as
commonly found for relaxed and well-concentrated clusters. Using the many
systems uncovered here we derive credible critical curves and Einstein radii
for different source redshifts. For a source at $z_{s}\simeq2.5$, the critical
curve encloses a large area with an effective Einstein radius of
$\theta_{E}=28\pm3\arcsec$, and a projected mass of $1.34\pm0.15\times10^{14}
M_{\odot}$. From the current understanding of structure formation in
concordance cosmology, these values are relatively high for clusters at
$z\sim0.5$, so that detailed studies of the inner mass distribution of clusters
such as MACS J1206.2-0847 can provide stringent tests of the $\Lambda$CDM
paradigm.
