Observation of a very massive galaxy cluster at z=0.76 in SRG/eROSITA
  all-sky survey
  The results of multiwavelength observations of the very massive galaxy
cluster SRGe CL2305.2-2248 detected in X-rays during the first SRG/eROSITA
all-sky survey are discussed. This galaxy cluster was also detected earlier in
microwave band through the observations of Sunyaev-Zeldovich effect in South
Pole Telescope (SPT-CL J2305-2248), and in Atacama Cosmological Telescope
(ACT-CL J2305.1-2248) surveys. Spectroscopic redshift measurement, $z=0.7573$,
was measured at the Russian 6-m BTA telescope of SAO RAS, in good agreement
with its photometric estimates, including a very accurate one obtained using
machine learning methods. In addition, deep photometric measurements were made
at the Russian-Turkish 1.5-m telescope (RTT150), which allows to study cluster
galaxies red sequence and projected galaxies distribution. Joint analysis of
the data from X-ray and microwave observations show that this cluster can be
identified as a very massive and distant one using the measurements of its
X-ray flux and integral comptonization parameter only. The mass of the cluster
estimated according to the eROSITA data is $M_{500}=(9.0\pm2.6)\cdot10^{14}\,
M_\odot$. We show that this cluster is found among of only several dozen of the
most massive clusters in the observable Universe and among of only a few the
most massive clusters of galaxies at $z>0.6$.
