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 (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,0.1-2.4keV = 6.4_-0.3^+0.7 10⁴⁴ erg/s, for the bolometric luminosity L_X,bol = 1.6_-0.3^+0.5 × 10⁴⁵ 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_-1.6^+2.8 keV and m = 0.22_-0.22^+0.24m⊙ 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 rom a virialised state. In the same ROSAT/HRI image the X-ray emission of a background quasar (Z_Q ≈ 2) can clearly be identified. With a luminosity of L_X,0.1-2.4keV ≈ 1.4 × 10⁴⁵ erg/s (which is necessarily affected by gravitational lensing) it belongs to he 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 often. We try to identify Ws source from deep optical data. Our best candidate is a blue compact object, possibly an AGN.