Nonthermal emission from clusters of galaxies
Received 21 April 2000 / Accepted 31 August 2000
We demonstrate that fast moving halos in clusters of galaxies provide a plausible source of (extended) nonthermal X-ray and -ray emission. This follows from the fact that accelerated particles are produced in their bow shocks. A substantial fraction of a virialized ensemble of halos can have moderate supersonic velocities outside the central region of a rich cluster. Dark matter halos of supersonic and superalfvenic velocity will create collisionless bow shocks of moderate Mach number 2, due to electromagnetic interaction between comoving baryonic matter and the hot intra-cluster gas. We present kinetic modelling of nonthermal electron injection, acceleration and propagation in such systems and conclude that the halos are efficient electron accelerators and sources of hard X-ray and -ray emission. Inverse-Compton radiation, bremsstrahlung, and synchrotron radiation by these electrons produce spectra that are in quantitative agreement with e.g. the hard X-ray and radio emission observed from the Coma cluster. Moreover, the relative increase of hard X-ray emission outside the central 0.3-0.4 Mpc of the cluster A2199, as observed by BeppoSAX , can be understood in this framework. Spatially resolved hard X-ray and -ray spectra of clusters (e.g. INTEGRAL ) can determine the contribution of the energetic lepton component generated by supersonic halo motions to the observed nonthermal emission, thus constraining the energetic nuclear component.
Key words: acceleration of particles radiation mechanisms: non-thermal galaxies: clusters: general gamma rays: theory radio continuum: galaxies X-rays: galaxies
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© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000