NED Abstract 
Copyright by European Southern Observatory (ESO).
Reproduced by permission

1999A&A...344..409E

Non-thermal origin of the EUV and HEX excess emission of the Coma cluster - 
the nature of the energetic electrons.

ENSSLIN T.A., LIEU R., BIERMANN P.L.

Received 20 August 1998/Accepted 14 January 1999

Abstract: The excess in extreme-ultraviolet (EUV) radiation and the 
recently discovered high energy X-ray (HEX) excess from the Coma cluster 
may be modeled using fewer parameters than in a thermal gas scenario, yet 
equally satisfactorily, by power law spectra. Their origin could therefore 
be inverse-Compton (IC) emission by relativistic electrons. The scattered 
background photon field can either be the cosmic microwave background (CMB) 
or the starlight of the elliptical galaxies within the cluster. 
   For the EUV excess both possibilities are consistent with the present 
data. If the EUV excess is due to CMB scattering, a strongly inhomogeneous 
magnetized intra-cluster medium (ICM) is required, in which the density of 
the IC scattering relativistic electrons is anticorrelated with the 
magnetic field. This could be understood if the electrons were accelerated 
during a major merger event within the last 2Gyr and cooled afterwards in 
the inhomogeneous fields. If the EUV excess is due to scattered starlight, 
a population of relativistic, very low energy electrons has to be present, 
which would have a high energy density. In order to survive Coulomb losses, 
these electrons have to be separated from the dense thermal cluster gas by 
confining magnetic fields. Such a second component of the ICM could be 
remnant radio plasma left over from the epoch of violent quasar activity, 
which did not mix with the ICM. The observed narrow radial profile of the 
EUV excess emission is a natural consequence of this model due to the 
narrow profile of the photon distribution. Both models favor therefore very 
inhomogeneous magnetic field and relativistic electron distributions. 
   The IC models for the HEX excess require implausible conditions. CMB 
scattering leads to a mainly unmagnetized ICM, in contradiction to Faraday 
rotation measurements. Starlight IC scattering electrons would overproduce 
EUV photons due to simultaneously CMB scattering. We propose that the 
observed HEX excess is due to bremsstrahlung of a small high energy power-
law tail of the mainly thermal ICM electron distribution. Such a tail is 
expected since some degree of turbulence has to be present within the ICM, 
which naturally accelerates electrons out of the thermal pool.

Keywords: radiation mechanisms: non-thermal - galaxies: clusters: 
individual: Coma cluster - galaxies: intergalactic medium - galaxies: 
magnetic fields - ultraviolet: galaxies - X-rays: galaxies