Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters

We present constraints on the mean dark energy density, Omega_X and dark energy equation of state parameter, w_X, based on Chandra measurements of the X-ray gas mass fraction in 26 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.07 -1, the same data require w_X<-0.7 at 95 per cent confidence. Similar results on the mean matter density and dark energy equation of state parameter, Omega_m=0.24\pm0.04 and w_X=-1.20^{+0.24}_{-0.28}, are obtained by replacing the CMB data with standard priors on the Hubble constant and mean baryon density and assuming a flat geometry.