EDP Sciences
Free access
Volume 508, Number 2, December III 2009
Page(s) 599 - 602
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361/200913177
Published online 15 October 2009
A&A 508, 599-602 (2009)
DOI: 10.1051/0004-6361/200913177

Research Note

Constraining relativistic protons and magnetic fields in galaxy clusters through radio and $\mathsf{\gamma}$-ray observations: the case of A2256

G. Brunetti

INAF - Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
    e-mail: brunetti@ira.inaf.it

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 $\alpha$ $\geq$ 1.5-1.6 $(F(\nu) \propto \nu^{-\alpha})$, 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 $\alpha$ = 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 $\gamma$-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, $\alpha$ ~ 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 $\leq$10-15 $\mu$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