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.