ROSAT/PSPC observation of the distant cluster CL0939+472

We present results and an analysis of a ROSAT PSPC pointed observation of the galaxy cluster CL0939+472. This very rich and very distant cluster is known to contain an unusually high fraction of blue and E+A galaxies, and a lot of merging of galaxies is going on. The X-ray properties we find are also quite unusual. The luminosity in the ROSAT band (0.1-2.4 keV) is $L_X = (6.7\pm 0.3) \times 10^{44}$erg/s, rather on the low side for such a rich cluster. We find a temperature of 2.9$_{-0.8}^{+1.3}$ keV, which is also relatively low, but has also a large uncertainty. The cluster shows indications of substructure, which results in a high value for the core radius and a steep drop off outside the core, if one fits a spherically symmetric $\beta$-model. The lower limit on the mass inside a radius of $1.0 h_{50}^{-1}$Mpc is $2.6 \times 10^{14} \msol$, and the upper limit on the gas mass fraction is 50\%. All the unusual properties of this cluster can be explained if in fact we see the merging of two clusters of roughly equal mass, which is also supported by recent optical observations.