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 ∼ 1014− 1015 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 non-orthogonal 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 ∼5–40 keV. We discuss this effect in light of the spectra from SGR 1806 — 20.