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 represents hitherto the strongest effect of its kind. Emission in the ~0.2-0.4 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 signal limit of the cluster. Our analysis of ROSAT PSPC observations reveal that the soft excess can be interpreted as either a thermal or nonthermal effect, and the high data quality allows us to place tight constraints on the two currently competing models. However, each model now implies that major revisions to our understanding of clusters of galaxies are needed: either "warm" gas masses in similar amounts to the hot gas or relativistic particles in or above equipartition with the hot phase appear to be unavoidable.