  While the photon forward-scattering amplitude on free magnetic dipoles (e.g.
free neutrons) vanishes, the nucleon magnetic moments still contribute
significantly to the photon dispersion relation in a supernova (SN) core where
the nucleon spins are not free due to their interaction. We study the frequency
dependence of the relevant spin susceptibility in a toy model with only
neutrons which interact by one-pion exchange. Our approach amounts to
calculating the photon absorption rate from the inverse bremsstrahlung process
gamma n n --> n n, and then deriving the refractive index n_refr with the help
of the Kramers-Kronig relation. In the static limit (omega --> 0) the
dispersion relation is governed by the Pauli susceptibility chi_Pauli so that
(n_refr)^2-1 approx chi_Pauli > 0. For omega somewhat above the neutron
spin-relaxation rate Gamma_sigma we find (n_refr)^2-1< 0, and for omega >>
Gamma_sigma the photon dispersion relation acquires the form
omega^2-k^2=(m_gamma)^2. An exact expression for the "transverse photon mass"
m_gamma is given in terms of the f-sum of the neutron spin autocorrelation
function; an estimate is (m_gamma)^2 approx chi_Pauli T Gamma_sigma. The
dominant contribution to n_refr$ in a SN core remains the electron plasma
frequency so that the Cherenkov processes gamma nu <--> nu remain forbidden for
all photon frequencies.
