The first detailed X-ray observations of high-redshift, optically selected clusters:: XMM-Newton results for Cl 1324+3011 at z=0.76 and Cl 1604+4304 at z=0.90

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 similar to0.5 h(70)(-1) Mpc. 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 greater than or similar to0.4, both clusters are modest X-ray emitters with bolometric luminosities of only L-X(bol) = (1.4-2.0) x 10(44) h(70)(-2) ergs s(-1). We measure gas temperatures of T = 2.88(-0.49)(+0.71) keV for Cl 1324 + 3011 and 2.51(-0.69)(+1.05) keV 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 greater than or equal to 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for z = 0.5 their measured velocity dispersions (sigma). The clusters are cooler by a factor of 2-9 compared to the local sigma-T relation. We briefly discuss the possible explanations for these results.