Photon splitting in gamma-ray pulsars

Magnetic photon splitting is an exotic QED process which becomes important in fields approaching the quantum critical value of 4.41 10(13) Gauss. Its principal effect is to degrade photon energies below the threshold for magnetic single photon pair production. In this paper, we consider the importance of photon splitting in gamma-ray pulsars, specifically focusing on PSR 1509-58, because of its large spin-down field (B = 3 10(13) Gauss). We present calculations of the splitting attenuation length, including the influence of the dipole field and curved spacetime geometry of a neutron star magnetosphere. Test photons originate at different locations on the neutron star surface, initially propagating parallel to the field lines. It is found that splitting prevents gamma-rays emitted from a polar cap of 5 degrees-10 degrees from escaping the magnetosphere when their energy exceeds about 30-50 MeV in fields similar to 10(13) Gauss. Comparison with attenuation lengths for pair creation indicates that splitting is more important in polar cap models for PSR 1509-58 in the 10-200 MeV range: splitting suppresses the creation of pairs and inhibits escape of any near-surface emission above about 10-20 MeV, depending on polar cap size. General relativistic effects on photon trajectories and on the magnetic field decrease the escape energies significantly.