XMM- Newton observations are presented for the z = 0.83 
cluster of galaxies MS 1054 - 0321 , the highest redshift cluster in the Einstein Extended Medium Sensitivity Survey ( EMSS ) .
The temperature inferred by the XMM- Newton data , T = 7.2 ^ { +0.7 } _ { -0.6 } keV , is much lower than the temperature previously reported from ASCA data , T = 12.3 ^ { +3.1 } _ { -2.2 } keV ( Donahue et al .
1998 ) , and a little lower than the Chandra temperature , T = 10.4 ^ { +1.7 } _ { -1.5 } keV , determined by Jeltema et al .
2001 .
The discrepancy between the newly derived temperature and the previously derived temperatures is discussed in detail .
If one allows the column density to be a free parameter , then the best fit temperature becomes T = 8.6 ^ { +1.2 } _ { -1.1 } keV , and the best fit column density becomes N _ { H } = 1.33 ^ { +0.15 } _ { -0.14 } \times 10 ^ { 20 } 
atoms cm ^ { -2 } .
The iron line is well detected in the XMM- Newton spectrum with a value for the abundance of Z = 0.33 ^ { +0.19 } _ { -0.18 } Z _ { \sun } , in very good agreement with previous determinations .
The derived XMM X-ray luminosity for the overall cluster in the 2–10 keV energy band is L _ { X } = ( 3.81 \pm 0.19 ) \times 10 ^ { 44 } h ^ { -2 } erg s ^ { -1 } 
while the bolometric luminosity is L _ { BOL } = ( 8.05 \pm 0.40 ) \times 10 ^ { 44 } h ^ { -2 } erg s ^ { -1 } .
The XMM- Newton data confirm the substructure in the cluster X-ray morphology already seen by R OSAT and in much more detail by Chandra .
We find that only two of the three clumps detected in the weak lensing mass reconstruction image are visible in X-rays , as already noted by [ ] .
The central weak lensing clump is coincident with the main cluster component and has a temperature T = 8.1 ^ { +1.3 } _ { -1.2 } keV .
The western weak lensing clump coincides with the western X-ray component which is much cooler with a temperature T = 5.6 ^ { +0.8 } _ { -0.6 } keV .
The optically measured velocity dispersion , obtained from 145 cluster redshifts , is consistent with the velocity dispersion expected from the \sigma _ { V } - T _ { X } relationship once the XMM- Newton temperature is used .
MS 1054 - 0321 fits well in the \sigma \propto T ^ { 1 / 2 } 
correlation available in the literature and derived from information collected for all clusters at redshfits of z \leq 1.27 known today and with a measured X-ray temperature .
The cluster temperature seems to be commensurate with the predictions from its X-ray luminosity from the L _ { X } - T _ { X } relation of local clusters .
Given the newly determined temperature , MS 1054 - 0321 is no longer amongst the hottest clusters known .