Discovery of an X-ray-luminous galaxy cluster at z=1.4

We report the discovery of a massive, X-ray - luminous cluster of galaxies at z = 1.393, the most distant X-ray - selected cluster found to date. XMMUJ2235.3 - 2557 was serendipitously detected as an extended X-ray source in an archival XMM-Newton observation of NGC 7314. VLT FORS2 R- and z-band snapshot imaging reveals an overdensity of red galaxies in both angular and color spaces. The galaxy enhancement is coincident in the sky with the X-ray emission; the cluster red sequence at R - z similar or equal to 2.1 identifies it as a high-redshift candidate. Subsequent FORS2 multiobject spectroscopy unambiguously confirms the presence of a massive cluster based on 12 concordant redshifts in the interval 1.38 < z < 1.40. The preliminary cluster velocity dispersion is 762 +/- 265 km s(-1). VLT ISAAC K-s- and J-band images underscore the rich distribution of red galaxies associated with the cluster. Based on a 45 ks XMM-Newton observation, we find that the cluster has an aperture-corrected unabsorbed X-ray flux of f(x) = (3.6 +/- 0.3) x 10(-14) ergs cm(-2) s(-1), a rest-frame X-ray luminosity of L-X = (3.0 +/- 0.2) x 10(44) h(70)(-2)ergs s(-1) (0.5 - 2.0 keV), and a temperature of keV), and a temperature of kT = 6.0(-1.8)(+2.5) keV. Though XMMU J2235.3 - 2557 is likely the first confirmed z>1 cluster found with XMM-Newton, the relative ease and efficiency of discovery demonstrates that it should be possible to build large samples of clusters through the joint z>1 use of X-ray and large ground-based telescopes.