Sersic 159-03: Discovery of the brightest soft X-ray excess emitting cluster of galaxies

The soft X-ray excess emission in the southern cluster Sersic 159-03 repres ents hitherto the strongest effect of its kind. Emission in the similar to0 .2-0.4 keV passband is detected far in excess of the expected contribution from the hot phase of the intracluster medium and extends to the X-ray sign al limit of the cluster. Our analysis of ROSAT PSPC observations reveal tha t the soft excess can be interpreted as either a thermal or nonthermal effe ct, and the high data quality allows us to place tight constraints on the t wo currently competing models. However, each model now implies that major r evisions to our understanding of clusters of galaxies are needed: either "w arm" gas masses in similar amounts to the hot gas or relativistic particles in or above equipartition with the hot phase appear to be unavoidable.