2004ApJ...601L...9L

We present the first detailed X-ray observations of optically selected clusters at high redshift. Two clusters, Cl 1324+3011 at z=0.76 and Cl 1604+4304 at z=0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of ∼0.5 h^–1₇₀Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similar to other optically selected clusters at redshifts of z≳0.4, both clusters are modest X-ray emitters with bolometric luminosities of only L^bol_X=1.4-2.0x10⁴⁴ h^–2₇₀ergs/s. We measure gas temperatures of T=2.88^+0.71_–0.49keV for Cl 1324+3011 and 2.51^+1.05_–0.69keV for Cl 1604+4304. The X-ray properties of both clusters are consistent with the high-redshift L_X-T relation measured from X-ray-selected samples at z≥0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for their measured velocity dispersions (σ). The clusters are cooler by a factor of 2-9 compared to the local σ-T relation. We briefly discuss the possible explanations for these results.