An X-ray-selected galaxy cluster at z=1.26

We report the discovery of an X-ray-luminous galaxy cluster at z = 1.26. RX J0848.9 + 4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray emission, with a significant excess of red objects with J-K colors typical of elliptical galaxies at z > 1. Spectroscopic observations at the Reck II Telescope have secured six galaxy redshifts in the range 1.257 < z < 1.267 ([z] = 1.261) within a 35 " radius around the peak X-ray emission. This system lies only 4.'2 away (54.0 h(50)(-1) comoving Mpc, q(0) = 0.5) from the galaxy cluster ClG J0848 + 4453, which was identified at z = 1.273 in a near-IR field galaxy survey and is also known to be X-ray luminous. Assuming that the X-ray emission is entirely due to hot intracluster gas, both these systems have similar rest-frame luminosities L-X approximate to 1 x 10(44) ergs s(-1) (0.5-2.0 keV band). In combination with our spectrophotometric data for the entire 30 arcmin(2) field, this suggests the presence of a superstructure consisting of two collapsed, possibly virialized, clusters, the first detected to date at z > 1.