X-ray and optical observations of three clusters of galaxies: Abell 901, Abell 1437, and Abell 3570

We analyse three clusters of galaxies, Abell 901 (z = 0.17), Abell 1437 (z = 0.13) and Abell 3570 (z = 0.037). They have low to intermediate X-ray flu xes and an irregular morphology in the ROSAT All-Sky Survey (RASS). These c lusters are chosen to test the abilities and limitations of the RASS in ter ms of cluster fluxes and cluster morphologies. Therefore some "worst" cases are used here. X-ray observations with the ROSAT/HRI and optical spectrosc opic observations are carried out. The ROSAT/HRI observations, which have a much better spatial resolution than the RASS, reveal in two of the three c ases a significantly different morphology than seen in the RASS. The reason s are point sources which could not be resolved in the RASS and were theref ore confused with the cluster emission. For A3570 we could confirm the rela xed state of the cluster by the optical determination of a small velocity d ispersion. In the cluster with the lowest flux (Abell 901) the countrate me asurement is strongly affected by point sources, in the tno other cases the countrate measurements of the RASS are reliable, i.e. they are reproduced by the ROSAT/HRI measurement. We conclude that for clusters with a flux of a few times 10(-12) erg/cm(2)/s or smaller, which show at the same time a n on-relaxed morphology the flux measurement of the RASS can be seriously aff ected by fore- or background sources. We point out that an all-sky survey o f a second ABRIXAS mission would provide a much clearer source distinction for low-flux clusters and thus a much improved countrate determination.