Origin of magnetar hard X-ray tails

Determine the physical origin of the hard X-ray emission component in magnetars by distinguishing among thermal bremsstrahlung from relativistic electrons in neutron-star surface layers, synchrotron emission from electron–positron pairs accelerated in a twisted magnetosphere, and resonant Compton up-scattering by relativistic charges.

Background

Magnetars exhibit hard X-ray tails extending from approximately 10 keV to several hundred keV, mostly outside the 2–8 keV band of IXPE but within the proposed 2–35 keV band of EXPO. The paper identifies the physical origin of this hard component as a major unresolved issue in neutron-star astrophysics.

The competing emission mechanisms make different predictions for X-ray polarization. Broadband X-ray polarimetry is therefore presented as a means of discriminating between these models and establishing the mechanism responsible for the hard tails.