  We use cosmological gas dynamic simulations to investigate the accuracy of
galaxy cluster mass estimates based on X-ray observations. The experiments
follow the formation of clusters in different cosmological models and include
the effects of gravity, pressure gradients, and hydrodynamical shocks. A subset
of our ensemble also allows for feedback of mass and energy from galactic winds
into the intracluster medium. We find that mass estimates based on the
hydrostatic, isothermal beta-model are remarkably accurate when evaluated at
radii where the cluster mean density is between 500-2500 times the critical
density. Applied to 174 artificial ROSAT images constructed from the
simulations, the distribution of the estimated-to-true mass ratio is nearly
unbiased and has a standard deviation of 14-29%. The scatter can be
considerably reduced (to 8-15%) by using an alternative mass estimator that
exploits the tightness of the mass-temperature relation found in the
simulations. The improvement over beta-model estimates is due to the
elimination of the variance contributed by the gas outer slope parameter. We
discuss these findings and their implications for recent measurements of
cluster baryon fractions.
