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 modeling. One of the most popular classes of models
involves strongly magnetised neutron stars, with surface dipole fields B~
10^{14}-10^{15} Gauss. In such strong magnetic fields, many otherwise
negligible processes can play an important role. Here we consider the effects
of vacuum polarisation on Compton scattering. Vacuum polarisation introduces a
characteristic density-dependent photon frequency at which the normal modes of
polarisation become nonorthogonal and the mean free path of photons decreases
sharply. Our analytic results and Monte Carlo simulations of photon propagation
through a magnetised plasma show that this effect leads, under a wide range of
physical conditions, to a broad absorption-like feature in the energy range ~5
keV---40 keV. We discuss this effect in light of the spectra from SGR 1806-20.Comment: 13 pages Latex , uses mn.sty. Submitted to MNRA
