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Astron. Astrophys. 344, 409-420 (1999)

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

Torsten A. Enßlin 1, Richard Lieu 2 and Peter L. Biermann 1

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany (ensslin; plbiermann@mpifr-bonn.mpg.de)
2 Department of Physics, University of Alabama, Huntsville, AL 35899, USA (lieur@cspar.uah.edu)

Received 20 August 1998 / Accepted 14 January 1999


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 2 Gyr 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.

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

© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999