Resonant Inverse Compton Scattering Spectra from Highly-magnetized Neutron Stars

TL;DR

This paper models the resonant inverse Compton scattering process in magnetar magnetospheres, providing detailed spectra and polarization predictions that align with observed hard X-ray emissions, and introduces a new QED scattering formulation.

Contribution

It presents a novel, physically accurate formulation of the QED Compton scattering cross section in strong magnetic fields for modeling magnetar X-ray spectra.

Findings

Electrons below 15 MeV emit mainly below 250 keV.

Spectra are sensitive to viewing geometry and electron energy.

Polarization predictions can aid future observational constraints.

Abstract

Hard, non-thermal, persistent pulsed X-ray emission extending between 10 keV and $\sim 150$ keV has been observed in nearly ten magnetars. For inner-magnetospheric models of such emission, resonant inverse Compton scattering of soft thermal photons by ultra-relativistic charges is the most efficient production mechanism. We present angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons injected in inner regions of magnetar magnetospheres, calculated using collisional integrals over field loops. Our computations employ a new formulation of the QED Compton scattering cross section in strong magnetic fields that is physically correct for treating important spin-dependent effects in the cyclotron resonance, thereby producing correct photon spectra. The spectral cut-off energies are sensitive to the choices of observer viewing geometry, electron Lorentz factor, and scattering kinematics. We find that electrons with energies $\lesssim 15$ MeV will emit most of their radiation below 250 keV, consistent with inferred turnovers for magnetar hard X-ray tails. More energetic electrons still emit mostly below 1 MeV, except for viewing perspectives sampling field line tangents. Pulse profiles may be singly- or doubly-peaked dependent upon viewing geometry, emission locale, and observed energy band. Magnetic pair production and photon splitting will attenuate spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The resonant Compton spectra are strongly polarized, suggesting that hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope, can prove central to constraining model geometry and physics.