We report the discovery of an X-ray luminous galaxy cluster at z = 1.26 .
RXJ0848.9+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 Keck II telescope have secured 6 galaxy redshifts in the range 1.257 < z < 1.267 ( < z > = 1.261 ) , within a 35 arcsec radius around the peak X–ray emission .
This system lies only 4.2 arcmin away ( 5.0 { h } ^ { -1 } _ { 50 } comoving Mpc , q _ { 0 } = 0.5 ) from the galaxy cluster ClG J0848+4453 , which was identified by Stanford et al .
( 1997 ) 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 intra-cluster gas , both these systems have similar rest frame luminosities L _ { X } \approx 1 \times 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 .