Abstract.

The X-ray luminosities of the Einstein Extended Medium Sensitivity Survey (EMSS) clusters of galaxies with redshifts 0.3$<z<$0.6 are remeasured using ROSAT PSPC data. It is found that the new luminosities are on average 1.18$\pm$0.08 times higher than previously measured but that this ratio depends strongly on the X-ray core radii we measure. For the clusters with small core radii, in general we confirm the EMSS luminosities, but for clusters with core radii $>$250 kpc (the constant value assumed in the EMSS), the new luminosities are 2.2$\pm$0.15 times the previous measurements. The X-ray luminosity function (XLF) at $0.3 < z < 0.6$ is recalculated and is found to be consistent with the local XLF. The constraints on the updated properties of the 0.3$<z<$0.6 EMSS sample, including a comparison with the number of clusters predicted from local XLFs, indicate that the space density of luminous, massive clusters has either not evolved or has increased by a small factor $\sim$2 since $z=0.4$. The implications of this result are discussed in terms of constraints on the cosmological parameter $\Omega_{0}$.