Electron-positron flows around magnetars

TL;DR

This paper models electron-positron plasma flows in magnetar magnetospheres, revealing a self-organized, radiatively locked outflow with implications for high-energy and radio emissions.

Contribution

It introduces a self-consistent model of plasma flow in magnetar magnetospheres considering radiation interactions and electric fields, highlighting the formation of a radiatively locked outflow.

Findings

The plasma flow self-organizes into a radiatively locked outflow with decreasing Lorentz factor.

An extended ultra-relativistic zone where plasma marginally scatters thermal photons.

Outer zone where plasma decelerates, accumulates, and reflects thermal photons.

Abstract

The twisted magnetospheres of magnetars must sustain a persistent flow of electron-positron plasma. The flow dynamics is controlled by the radiation field around the hot neutron star. The problem of plasma motion in the self-consistent radiation field is solved using the method of virtual beams. The plasma and radiation exchange momentum via resonant scattering and self-organize into the "radiatively locked" outflow with a well-defined, decreasing Lorentz factor. There is an extended zone around the magnetar where the plasma flow is ultra-relativistic; its Lorentz factor is self-regulated so that it can marginally scatter thermal photons. The flow becomes slow and opaque in an outer equatorial zone, where the decelerated plasma accumulates and annihilates; this region serves as a reflector for the thermal photons emitted by the neutron star. The e+- flow carries electric current, which is sustained by a moderate induced electric field. The electric field maintains a separation between the electron and positron velocities, against the will of the radiation field. The two-stream instability is then inevitable, and the induced turbulence can generate low-frequency emission. In particular, radio emission may escape around the magnetic dipole axis of the star. Most of the flow energy is converted to hard X-ray emission, which is examined in the accompanying paper.