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

The association of all three soft gamma-ray repeaters (SGRs) with supernovaremnants has established that SGRs are young neutron stars, and has given us astarting point for detailed modeling. One of the most popular classes of modelsinvolves strongly magnetised neutron stars, with surface dipole fields B~10^{14}-10^{15} Gauss. In such strong magnetic fields, many otherwisenegligible processes can play an important role. Here we consider the effectsof vacuum polarisation on Compton scattering. Vacuum polarisation introduces acharacteristic density-dependent photon frequency at which the normal modes ofpolarisation become nonorthogonal and the mean free path of photons decreasessharply. Our analytic results and Monte Carlo simulations of photon propagationthrough a magnetised plasma show that this effect leads, under a wide range ofphysical conditions, to a broad absorption-like feature in the energy range ~5keV---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