The influence of magnetic field geometry on magnetars X-ray spectra

TL;DR

This study investigates how realistic magnetic field geometries influence the X-ray spectra of magnetars, revealing that while plasma parameters mainly shape the spectrum, magnetic geometry significantly affects its angular dependence.

Contribution

The paper introduces the use of new force-free magnetic field solutions in Monte Carlo simulations to assess their impact on magnetar X-ray spectra, advancing beyond simple models.

Findings

Magnetic field geometry affects the angular dependence of spectra.

Plasma parameters primarily determine the spectral shape.

Realistic magnetic geometries influence observational signatures.

Abstract

Nowadays, the analysis of the X-ray spectra of magnetically powered neutron stars or magnetars is one of the most valuable tools to gain insight into the physical processes occurring in their interiors and magnetospheres. In particular, the magnetospheric plasma leaves a strong imprint on the observed X-ray spectrum by means of Compton up-scattering of the thermal radiation coming from the star surface. Motivated by the increased quality of the observational data, much theoretical work has been devoted to develop Monte Carlo (MC) codes that incorporate the effects of resonant Compton scattering in the modeling of radiative transfer of photons through the magnetosphere. The two key ingredients in this simulations are the kinetic plasma properties and the magnetic field (MF) configuration. The MF geometry is expected to be complex, but up to now only mathematically simple solutions (self-similar solutions) have been employed. In this work, we discuss the effects of new, more realistic, MF geometries on synthetic spectra. We use new force-free solutions in a previously developed MC code to assess the influence of MF geometry on the emerging spectra. Our main result is that the shape of the final spectrum is mostly sensitive to uncertain parameters of the magnetospheric plasma, but the MF geometry plays an important role on the angle-dependence of the spectra.