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Astron. Astrophys. 362, 886-894 (2000)

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Nonthermal emission from clusters of galaxies

A.M. Bykov 1, H. Bloemen 2 and Yu.A. Uvarov 1

1 A.F. Ioffe Institute for Physics and Technology, St. Petersburg, 194021, Russia
2 SRON Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

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 [FORMULA]-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 [FORMULA] 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 [FORMULA]-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 [FORMULA]-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

Send offprint requests to: A.M. Bykov (byk@astro.ioffe.rssi.ru)

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000