Activated Magnetospheres of Magnetars

TL;DR

This paper explores the theory and observational evidence of magnetospheric activity in magnetars, focusing on the formation of a shrinking current bundle, plasma interactions, and X-ray emission mechanisms.

Contribution

It introduces a new model describing the untwisting of magnetar magnetospheres and the resulting plasma dynamics and X-ray spectra, supported by recent observations and simulations.

Findings

Shrinking hot spots consistent with j-bundle behavior

High e+- multiplicity in the magnetosphere

X-ray spectra shaped by reflection and upscattering mechanisms

Abstract

Like the solar corona, the external magnetic field of magnetars is twisted by surface motions of the star. The twist energy is dissipated over time. We discuss the theory of this activity and its observational status. (1) Theory predicts that the magnetosphere tends to untwist in a peculiar way: a bundle of electric currents (the "j-bundle") is formed with a sharp boundary, which shrinks toward the magnetic dipole axis. Recent observations of shrinking hot spots on magnetars are consistent with this behavior. (2) Continual discharge fills the j-bundle with electron-positron plasma, maintaining a nonthermal corona around the neutron star. The corona outside a few stellar radii strongly interacts with the stellar radiation and forms a "radiatively locked" outflow with a high e+- multiplicity. The locked plasma annihilates near the apexes of the closed magnetic field lines. (3) New radiative-transfer simulations suggest a simple mechanism that shapes the observed X-ray spectrum from 0.1 keV to 1 MeV: part of the thermal X-rays emitted by the neutron star are reflected from the outer corona and then upscattered by the inner relativistic outflow in the j-bundle, producing a beam of hard X-rays.