Magnetar Magnetospheres under the Microscope  [slides]

Abstract

Magnetars are young neutron stars with high surface magnetic fields, exceeding around 10 TeraGauss. Pulsed, non-thermal quiescent X-ray emission extending between 10 keV to >150 keV has been observed in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and Fermi-GBM. This talk describes a model for the generation of such hard X-ray signals, wherein the emission is produced by resonant Compton upscattering of soft thermal photons from the neutron star surface. This mechanism is efficient due to the enhancement of the scattering cross section at the cyclotron resonance. Spectra that approximately match observations can be generated by emission volumes within about ten stellar radii of the magnetar surface. The observed/implied maximum energies of these hard X-ray signals may require that an attenuation process such as magnetic photon splitting be active. This prospect can be probed by future sensitive Compton telescopes with polarimetric capability, for example the AMEGO concept. Interestingly, key pulsation and spectral signatures can be used to constrain the viewing geometry and the angle between the magnetic and spin axes of magnetars, a diagnostic that may improve the ability to precisely determine the strengths of magnetar fields.