Planck early results. XXVI. Detection with Planck and confirmation by XMM-Newton of PLCK G266.6-27.3, an exceptionally X-ray luminous and massive galaxy cluster at z similar to 1

We present first results on PLCK G266.6-27.3, a galaxy cluster candidate detected at a signal-to-noise ratio of 5 in the Planck All Sky survey. An XMM-Newton validation observation has allowed us to confirm that the candidate is a bona fide galaxy cluster. With these X-ray data we measure an accurate redshift, z = 0.94 +/- 0.02, and estimate the cluster mass to be M-500 = (7.8 +/- 0.8) x 10(14) M-circle dot. PLCK G266.6-27.3 is an exceptional system: its luminosity of L-X[0.5-2.0 keV] = (1.4 +/- 0.05) x 10(45) erg s(-1) equals that of the two most luminous known clusters in the z > 0.5 universe, and it is one of the most massive clusters at z similar to 1. Moreover, unlike the majority of high-redshift clusters, PLCK G266.6-27.3 appears to be highly relaxed. This observation confirms Planck's capability of detecting high-redshift, high-mass clusters, and opens the way to the systematic study of population evolution in the exponential tail of the mass function.