Fractional polarization as a probe of magnetic fields in the intra-cluster medium A. Bonafede, F. Govoni, L. Feretti, M. Murgia, G. Giovannini and M. Brüggen A&A, 530 (2011) A24 Published online: 02 May 2011 DOI: 10.1051/0004-6361/201016298