X-RAY OBSERVATIONS OF THE RICH CLUSTER CL-0939+4713 AND DISCOVERY OF THE STRONGLY VARIABLE SOURCE RXJ0943.0+4701

Recent X-ray observations of the rich galaxy cluster CL 0939 + 4713 (o r Abell 851) at z = 0.41 with the ROSAT/HRI as well as with the ASCA/G IS 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 t he 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,L-0.1 - 2.4 keV = 6.4(-0.3)(+0.7) x 10(44 ) erg/s, for the bolometric luminosity minosity L-X,L-bol = 1.6(-0.3)( +0.5) x 10(45) erg/s. We perform spectral fits for the two ASCA instru ments and for the ROSAT/PSPC simultaneously. The most reliable numbers for the temperature and metallicity for the intracluster gas are T = 7.6(-1.6)(+2.8) keV and m = 0.22(-0.22)(+0.25)m(circle dot), respectiv ely. We find a relatively small total mass, a small gas mass ratio and a small iron mass to light ratio. These numbers together with relativ ely low luminosity for such an optically rich cluster and the pronounc ed substructure confirm that CL 0939 + 4713 is a young cluster still f ar away from a virialised state.