Hard X-ray Quiescent Emission in Magnetars via Resonant Compton Upscattering

TL;DR

This paper models the hard X-ray emission in magnetars through resonant Compton upscattering of relativistic electrons, showing how viewing angles and electron energies influence observed spectra, aligning with observed magnetar tails.

Contribution

It introduces a new formalism for QED Compton scattering in strong magnetic fields and applies it to model magnetar X-ray spectra considering different viewing angles.

Findings

Electrons below 15 MeV emit mostly below 250 keV, matching observed turnovers.

Higher energy electrons emit mainly below 1 MeV, with emission zones affecting pulse phases.

Spectral features depend critically on observer angle and electron Lorentz factor.

Abstract

Non-thermal quiescent X-ray emission extending between 10 keV and around 150 keV has been seen in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and Fermi-GBM. For inner magnetospheric models of such hard X-ray signals, inverse Compton scattering is anticipated to be the most efficient process for generating the continuum radiation, because the scattering cross section is resonant at the cyclotron frequency. We present hard X-ray upscattering spectra for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar magnetospheres. These model spectra are integrated over bundles of closed field lines and obtained for different observing perspectives. The spectral turnover energies are critically dependent on the observer viewing angles and electron Lorentz factor. We find that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the turnovers inferred in magnetar hard X-ray tails. Electrons of higher energy still emit most of the radiation below around 1 MeV, except for quasi-equatorial emission locales for select pulse phases. Our spectral computations use a new state-of-the-art, spin-dependent formalism for the QED Compton scattering cross section in strong magnetic fields.