  Using the South Pole Telescope (SPT), we have discovered the most massive
known galaxy cluster at z > 1, SPT-CL J2106-5844. In addition to producing a
strong Sunyaev-Zel'dovich effect signal, this system is a luminous X-ray source
and its numerous constituent galaxies display spatial and color clustering, all
indicating the presence of a massive galaxy cluster. VLT and Magellan
spectroscopy of 18 member galaxies shows that the cluster is at z =
1.132^+0.002_-0.003. Chandra observations obtained through a combined HRC-ACIS
GTO program reveal an X-ray spectrum with an Fe K line redshifted by z = 1.18
+/- 0.03. These redshifts are consistent with galaxy colors in extensive
optical, near-infrared, and mid-infrared imaging. SPT-CL J2106-5844 displays
extreme X-ray properties for a cluster, having a core-excluded temperature of
kT = 11.0^+2.6_-1.9 keV and a luminosity (within r_500) of L_X (0.5 - 2.0 keV)
= (13.9 +/- 1.0) x 10^44 erg/s. The combined mass estimate from measurements of
the Sunyaev-Zel'dovich effect and X-ray data is M_200 = (1.27 +/- 0.21) x 10^15
M_sun. The discovery of such a massive gravitationally collapsed system at high
redshift provides an interesting laboratory for galaxy formation and evolution,
and is a powerful probe of extreme perturbations of the primordial matter
density field. We discuss the latter, determining that, under the assumption of
LambdaCDM cosmology with only Gaussian perturbations, there is only a 7% chance
of finding a galaxy cluster similar to SPT-CL J2106-5844 in the 2500 deg^2 SPT
survey region, and that only one such galaxy cluster is expected in the entire
sky.
