Phase-resolved Spectropolarimetric Models of Magnetar Hard X-ray Emission with QED Photon Splitting Attenuation

Non-thermal, pulsed hard X-ray tails are a signature persistent emission component from magnetars. These tails extend between 10 keV and about 100–200 keV, and have been detected in around ten magnetars to date. The prevailing paradigm is that this component is generated via resonant inverse Compton scattering by relativistic electrons of soft thermal photons propagating from the stellar surface into the inner magnetosphere. Recent work by our group has presented template angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons, calculated using collisional integrals over field loops. This advanced spectral modeling is extended by integrating these templates over substantial magnetospheric volumes, delivering spectra and pulse profiles, and exploring the polarization characteristics of the emission. Principal QED opacity processes are included, namely magnetic pair production and photon splitting, which attenuate spectra at hard X-ray and higher energies. Opacity to photon splitting can be significant above 100 keV, introducing strong polarization that can be probed by future hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope like AMEGO and COSI.