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

Abstract

The soft X-ray excess emission in the southern cluster Sersic 159-03 represents hiterto 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 intra-cluster 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 non-thermal effect, and the high data quality allows to place tight constraints on the two currently competing models. However, each model now implies major revisions to our understanding of clusters of galaxies: 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.