Spectral effects of the vacuum resonance in soft gamma-ray repeaters

The association of all three soft gamma-ray repeaters (SGRs) with supernova remnants has established that SGRs are young neutron stars, and has given us a starting point for detailed modelling. One of the most popular classes of models involves strongly magnetized neutron stars, with surface dipole fields B similar to 10(14)-10(15) G. In such strong magnetic fields, many otherwise negligible processes can play an important role. Here we consider the effects of vacuum polarization on Compton scattering. Vacuum polarization introduces a characteristic density-dependent photon frequency at which the normal modes of polarization become nonorthogonal and the mean free path of photons decreases sharply. Our analytic results and Monte Carlo simulations of photon propagation through a magnetized plasma show that this effect leads, under a wide range of physical conditions, to a broad absorption-like feature in the energy range similar to 5-40 keV. We discuss this effect in light of the spectra from SGR 1806-20.