EDP Sciences
Free access
Volume 424, Number 3, September IV 2004
Page(s) 773 - 778
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20041174

A&A 424, 773-778 (2004)
DOI: 10.1051/0004-6361:20041174

The Coma Cluster at $\gamma$-ray energies: Multifrequency constraints

A. Reimer1, O. Reimer1, R. Schlickeiser1 and A. Iyudin2

1  Institut für Theoretische Physik IV, Ruhr-Universität Bochum, 44780 Bochum, Germany
    e-mail: afm@tp4.rub.de
2  Max-Planck Institut für extraterrestrische Physik, Gießenbachstraße, 85740 Garching, Germany

(Received 22 March 2004 / Accepted 1 June 2004)

The Coma cluster exhibits evidence of a high-energy non-thermal particle population. At frequencies >1 GHz recent radio halo observations confirm a significant spectral steepening of the volume-integrated emission. We calculate the volume-averaged high-energy spectrum due to inverse Compton scattering off the CMB radiation field and non-thermal bremsstrahlung according to an exponential cutoff in the electron spectrum as deduced from the radio observations. The synchrotron radiation from secondary pairs, created from the decay of charged mesons produced in hadronic pp-interactions, is found to set significant constraints on the energy content of relativistic hadrons in Coma. This limits the maximum flux at high energies. Our findings support a low ratio of relativistic hadron to thermal energy density. Predictions for Coma's high energy emission are discussed in the light of current and expected abilities of upcoming $\gamma$-ray instruments.

Key words: galaxies: clusters: individual: Coma -- gamma rays: theory -- radiation mechanisms: non-thermal

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