Magnetar X-ray Spectra: An Utramagnetic QED Approach

This thesis considers the generation of the bright, broadband and non-thermal X-ray spectra of magnetars. These spectra are very sensitive to the distribution of electrons and positrons in the current-carrying magnetic field tubes; previous approaches considered generating the non-thermal components by modifying a seed thermal spectrum with resonant cyclotron upscattering on ultrarelativistic outflows, as well as through bremsstrahlung and synchrotron emission. The present approach constructs a Monte Carlo model of this persistent emission, where both ``AXP-like'' and ``SGR-like'' spectra are generated self-consistently as a result of the interaction of a warm magnetospheric plasma layer and a cold, dense neutron star atmospheric surface layer. This approach is motivated in part by evidence that the magnetospheric currents are strongly localized. In order to construct this model, a range of QED rates and cross sections are reviewed and calculated in the ultramagnetic background, many for the first time. In particular, the high electron-positron scattering and bremsstrahlung rates as well as the highly kinematically modified rates of two-photon pair creation and annihilation lead to a physical situation that differs qualitatively from previous models, including unique potential observational signatures.Ph.D