<i>Planck</i>early results. XXVI. Detection with<i>Planck</i>and confirmation by<i>XMM-Newton</i>of PLCK G266.6–27.3, an exceptionally X-ray luminous and massive galaxy cluster at<i>z</i> ~  1

We present first results on PLCKG266.6−27.3, 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 isa 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 M500 = (7.8 ± 0.8) × 1014 M⊙. PLCKG266.6−27.3 is an exceptional system: its luminosity of LX [0.5−2.0 keV] = (1.4 ± 0.05) × 1045 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 ~ 1. Moreover, unlike the majority of high-redshift clusters, PLCKG266.6−27.3 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.