We analysed three clusters of galaxies with low to intermediate X-ray luminosities which show an irregular appearance in the ROSAT All-Sky Survey. As the confusion with fore- and background sources is increasingly critical with decreasing flux and increasing substructure of the cluster, we test the limitations of the RASS. With only three clusters, of course, this cannot be a statistical study, but it is meant to be a "worst-case'' study. We would like to stress that for higher fluxes source confusion is less important and that for clusters with these fluxes the RASS results are very reliable.
The results of the follow-up observations - X-ray observations with the ROSAT/HRI and optical spectroscopic observations - are summarized in Table 5. The ROSAT/HRI observations, which have a much better spatial resolution than the RASS, revealed in two of the three cases a different morphology than seen in the RASS. The reasons are point sources which could not be resolved in RASS and were therefore confused with the cluster emission. In one of the three clusters the countrate measurement is affected by the point sources, in the two other cases the countrate measurements of the RASS are reliable. The results for the three clusters are the following:
A new X-ray all-sky survey with a potential second ABRIXAS (Friedrich et al. 1996) mission would have several advantages to push this source confusion limit down.
I thank Chris Collins for introducing me into the secrets of optical spectra, Hans Böhringer for making the RASS images of the three clusters available to me, Peter Friedrich for providing the ABRIXAS numbers, and Phil James and Joachim Wambsganss for carefully reading the manuscript. It is a pleasure to thank Carlo Izzo for his most helpful EXSAS support. I acknowledge gratefully the hospitality of the Institut d'Estudis Espacials de Catalunya in Barcelona. During the stay there I was supported by the TMR grant ERB-FMGE CT95 0062 by CESCA-CEPBA.
Copyright The European Southern Observatory (ESO)