  Observations of relaxed, massive and distant clusters can provide important
tests of standard cosmological models e.g. using the gas mass fraction. We
study the very luminous, high redshift ($z=0.902$) galaxy cluster
ClJ120958.9+495352 using XMM-Newton data and measure the temperature profile
and cooling time to investigate the dynamical status with respect to the
presence of a cool core as well as global cluster properties. We use HST weak
lensing data to estimate its total mass and determine the gas mass fraction. We
perform a spectral analysis using an XMM-Newton observation of 15ks cleaned
exposure time. As the treatment of the background is crucial, we use two
different approaches to account for the background emission to verify our
results. We account for point-spread-function effects and deproject our results
to estimate the gas mass fraction of the cluster. We measure weak lensing
galaxy shapes from mosaic HST imaging and select background galaxies
photometrically in combination with imaging data from the William Herschel
Telescope. The X-ray luminosity of ClJ120958.9+495352 in the 0.1-2.4keV band
estimated from our XMM-Newton data is $L_X =
(13.4_{-1.0}^{+1.2})\times10^{44}$erg/s and thus it is one of the most X-ray
luminous clusters known at similarly high redshift. We find clear indications
for the presence of a cool core from the temperature profile and the central
cooling time, which is very rare at such high redshifts. Based on the weak
lensing analysis we estimate a cluster mass of
$M_\mathrm{500}/10^{14}M_\odot=4.4^{+2.2}_{-2.0}(\mathrm{stat.})\pm0.6(\mathrm{sys.})$
and a gas mass fraction of $f_\mathrm{gas,2500} = 0.11_{-0.03}^{+0.06}$ in good
agreement with previous findings for high redshift and local clusters.
