Speaker
Description
Magnetars are young neutron stars with high surface magnetic fields, exceeding $10^{13}$ Gauss. 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. For inner magnetospheric models of such hard X-ray signals, inverse resonant Compton upscattering of soft thermal photons from the neutron star surface is the most efficient process for generating the continuum radiation in high magnetic fields. Such upscattering emission is anticipated to exhibit strong polarization above around 30 keV that is pulse phase-dependent. These signatures define science agendas for future hard X-ray polarimeters and Compton telescopes. In this paper we present detailed model predictions of emission spectra and polarization signals, addressing prospects for measuring the spectral cutoffs with a future Compton telescope such as AMEGO. Phase-resolved, spectropolarimetric observations will be critical in assessing the zones of activation, as well as constraining the viewing geometry and the angle between the magnetic and spin axes of magnetars. Polarization measurements may also probe fundamental strong-field QED processes operating in the magnetar magnetospheres, potentially distinguishing between spectral cutoffs due to magnetic pair production or photon splitting. Thus, polarization probes of magnetar X-ray emission can provide insights into nature that are currently beyond the reach of current terrestrial experiments.
