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 Dee
p 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 sys
tem 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 nea
r-IR field galaxy survey and is also known to be X-ray luminous. Assuming t
hat 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 d
ata for the entire 30 arcmin(2) field, this suggests the presence of a supe
rstructure consisting of two collapsed, possibly virialized, clusters, the
first detected to date at z > 1.