Recent X-ray observations of the rich galaxy cluster CL 0939+4713 ( or Abell 851 ) at z = 0.41 with the ROSAT/HRI as well as with the ASCA/GIS and ASCA/SIS instruments are presented and analysed .
With the high resolution imaging data ( ROSAT/HRI ) we confirm and extend the earlier impression that the cluster has a lot of substructure .
Two maxima of the cluster emission are obvious in the images , each of them shows even some internal structure .
One of the subclusters can be nicely modeled with an elliptical model .
For the total luminosity of the cluster in the ROSAT band we obtain L _ { X, 0.1 - 2.4 keV } = 6.4 _ { -0.3 } ^ { +0.7 } \times 10 ^ { 44 } erg/s , for the bolometric luminosity L _ { X,bol } = 1.6 _ { -0.3 } ^ { +0.5 } \times 10 ^ { 45 } erg/s .
We perform spectral fits for the two ASCA instruments and for the ROSAT/PSPC simultaneously .
The most reliable numbers for the temperature and metallicity for the intracluster gas are T = 7.6 ^ { +2.8 } _ { -1.6 } keV and m = 0.22 ^ { +0.24 } _ { -0.22 } m _ { \odot } , respectively .
We find a relatively small total mass , a small gas mass ratio and a small iron mass to light ratio .
These numbers together with relatively low luminosity for such an optically rich cluster and the pronounced substructure confirm that CL 0939+4713 is a young cluster still far away from a virialised state .

In the same ROSAT/HRI image the X-ray emission of a background quasar ( z _ { Q } \approx 2 ) can clearly be identified .
With a luminosity of L _ { X, 0.1 - 2.4 keV } \approx 1.4 \times 10 ^ { 45 } erg/s ( which is necessarily affected by gravitational lensing ) it belongs to the X-ray brightest quasars .
A striking difference between the ROSAT/PSPC and the ROSAT/HRI observations ( taken almost five years apart ) is the fainting of one X-ray source ( RXJ0943.0+4701 ) by at least a factor of ten .
We try to identify this source from deep optical data .
Our best candidate is a blue compact object , possibly an AGN .