  We investigate a sample of 14 clusters of galaxies observed with XMM-Newton
in a search for soft X-ray excess emission. In five of these clusters a
significant soft excess is evident. This soft X-ray excess is compared with the
thermal emission from both the hot intracluster gas and any cooling (flow) gas
that may be present. A warm (kT=0.2 keV), extended (several Mpc), plasma
component is particularly clear in the outer parts of the cluster, where the
normal cluster X-ray emission is weak. This warm component causes both a
thermal soft X-ray excess at low energies (below 0.4-0.5 keV), as well as O VII
line emission with a redshift consistent with a cluster origin, and not easily
interpreted as Galactic foreground emission. The intensity of this component is
commensurate with what has been measured before with the ROSAT PSPC in the 1/4
keV band. We attribute this component to emission from intercluster filaments
of the Warm-Hot Intergalactic Medium in the vicinity of these clusters. For the
central regions of clusters the detection of lines in the soft X-ray spectrum
is more difficult, due to the predominance of the X-ray emitting hot plasma
there, hence we cannot discriminate between the thermal and nonthermal origin
of the soft excess, leaving several options open. These include thermal
emission from warm filaments seen in projection in front of or behind the
cluster center, thermal or nonthermal emission in the cluster core itself
related to magnetic reconnection, or Inverse Compton emission from the cosmic
microwave background on relativistic electrons.
