Constraining relativistic protons and magnetic fields in galaxy clusters through radio and -ray observations: the case of A2256G. Brunetti
INAF - Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
Received 25 August 2009 / Accepted 18 September 2009
Giant radio halos are the most relevant examples of diffuse synchrotron emission from galaxy clusters. A number of these sources have very steep spectra, of spectral index 1.5-1.6 , and are ideal targets for testing current models of the origin of the relativistic particles. A2256 hosts the nearest radio halo with a very steep spectrum, of = 1.61, and a very large population of relativistic protons in the cluster would be necessary if the halo were produced by synchrotron emission from secondary particles. In this case, the 0.1-1 GeV -ray luminosity is expected to be 10-20 times higher than that of clusters hosting radio halos of similar radio power at GHz frequencies but with spectra more typical of the presently observed halo population, ~ 1.2. Based on these assumptions, future FERMI/GLAST observations are expected to detect A2256, provided that the magnetic field in the central cluster region is 10-15 G. We show that this will provide a prompt test of hadronic models for the origin of radio halos, and complementary constraints on both the cluster magnetic field and the physics of particle acceleration mechanisms.
Key words: radiation mechanisms: non-thermal -- galaxies: clusters: general -- radio continuum: general -- gamma rays: theory
© ESO 2009