Simulating the soft X-ray excess in clusters of galaxies

The detection of an excess of soft X-ray or Extreme Ultraviolet (EUV)
radiation, above the thermal contribution from the hot intracluster
medium (ICM), has been a controversial subject ever since the initial
discovery of this phenomenon. We use a large-scale hydrodynamical
simulation of a concordance \LambdaCDM model, to investigate the
possible thermal origin of such an excess in a set of 20 simulated
clusters having temperatures in the range 1-7 keV. Simulated clusters
are analysed by mimicking the observational procedure applied to
ROSAT-PSPC data, which for the first time showed evidence for the soft
X-ray excess: we compare the low-energy (e.g. [0.2-1] keV) part of the
spectrum of each cluster with that predicted for a plasma having
temperature and metallicity as computed after weighting by the
emissivity in a harder band (e.g., [1-2] keV). For distances to the
cluster center 0.4< R/R_vir< 0.7 we detect a significant
excess in most of the simulated clusters, whose relative amount changes
from cluster to cluster and, for the same cluster, by changing the
projection direction. In about 30 per cent of the cases the soft X-ray
flux is measured to be at least 50 per cent larger than predicted by the
one-temperature plasma model. We find that this excess is generated in
most cases within the virialized regions of the cluster. It mainly comes
from low-entropy and high-density gas associated with merging sub-halos,
rather than from diffuse warm gas. Only in a few cases does the excess
arise from fore/background groups observed in projection, while no
evidence is found for a significant contribution from gas lying within
large-scale filaments. We compute the distribution of the relative soft
excess, as a function of the distance to the cluster center , and
compare it with the observational result by Bonamente et al.
(\cite{Bonamente03}) for the Coma cluster. As in the observations, we
find that the relative excess increases with the distance from the
cluster center, with no significant excess detected for
R<0.4R_vir. However, an excess as large as that reported
for the Coma cluster at scales 0.4⪉ R/R_vir⪉ 0.7 is
found to be rather unusual in our set of simulated clusters.