Photon Splitting in Magnetar Models of Soft Gamma Repeaters

The recent association of soft gamma repeaters (SGRs) with counterparts in other wavebands has sparked much interest in these sources. One of the recent models for these objects is that they originate in the environs of neutron stars with fields much stronger than the quantum critical field \teq{B_{cr}=4.413\times 10^{13}} Gauss. Near such neutron stars, dubbed magnetars, the exotic quantum process of magnetic photon splitting becomes prolific. Its principal effect is to degrade photon energies and thereby soften gamma-ray spectra from neutron stars; it has recently been suggested that splitting may be responsible for limiting the hardness of emission in SGRs, if these sources originate in neutron stars with supercritical surface fields. Seed photons in supercritical fields efficiently generate soft gamma-ray spectra, typical of repeaters. In this paper, the influence of the curved dipole field geometry of a neutron star magnetosphere on the photon splitting rate is investigated. The dependence of the attenuation length on the location and angular direction of the seed photons is explored.