Resonant Inverse Compton Scattering and Hard X-ray Emission in Magnetar Magnetospheres

TL;DR

This paper models resonant Compton scattering in magnetar magnetospheres to explain persistent hard X-ray emission, analyzing how plasma density and geometry influence observed spectra and polarization.

Contribution

It introduces detailed calculations of scattering opacity, spectrum, and polarization within the pair outflow framework, linking geometry to observational features.

Findings

Resonant cooling significantly alters plasma density in magnetospheres.

Equatorial twist models fit NuSTAR spectra under IXPE viewing geometry.

Polar-twist configurations are less consistent with observations.

Abstract

Magnetars are a subclass of neutron stars with ultra-strong surface magnetic fields. Some magnetars exhibit persistent hard X-ray emission, characterized by power-law tails with photon indices around 1--1.5, extending from ${\sim}$10 keV to several hundred keV. The leading explanation for this hard X-ray component is resonant Compton scattering, in which the thermal seed photons are upscattered by relativistic electron-positron pairs flowing along magnetic field lines in the magnetosphere. In this work, we adopt the pair outflow framework of the magnetar magnetosphere and calculate the resonant Compton scattering opacity, as well as the spectrum and polarization of the upscattered emission. We find that resonant cooling can substantially modify the magnetospheric plasma density and impose strong optical depth constraints on the hard X-ray emission regions. Under the viewing geometry inferred from IXPE, an equatorial twist near the stellar surface provides a viable configuration for the NuSTAR hard X-ray spectrum of 4U 0142+61, while a polar-twist geometry is disfavored. Joint spectral, timing, and polarimetric modeling will be essential for distinguishing between the magnetospheric scattering geometries and understanding the physical properties of the pair plasma.