A QED Model for Non-thermal Emission from SGRs and AXPs

TL;DR

This paper explores how quantum electrodynamics effects cause fast magnetohydrodynamic modes in magnetars to produce non-thermal electron-positron emissions, potentially explaining observed emissions from SGRs and AXPs.

Contribution

It introduces a model linking QED effects to non-thermal emissions in magnetar magnetospheres, expanding understanding of their radiation mechanisms.

Findings

Non-thermal electron-positron distributions can form far from the star.

Fast mode breakdown can produce observable non-thermal radiation.

The model explains emissions from SGRs and AXPs without fireball formation.

Abstract

Previously, we showed that, owing to effects arising from quantum electrodynamics (QED), magnetohydrodynamic fast modes of sufficient strength will break down to form electron-positron pairs while traversing the magnetospheres of strongly magnetised neutron stars. The bulk of the energy of the fast mode fuels the development of an electron-positron fireball. However, a small, but potentially observable, fraction of the energy ($\sim 10^{33}$ ergs) can generate a non-thermal distribution of electrons and positrons far from the star. In this paper, we examine the cooling and radiative output of these particles. We also investigate the properties of non-thermal emission in the absence of a fireball to understand the breakdown of fast modes that do not yield an optically thick pair plasma. This quiescent, non-thermal radiation associated with fast mode breakdown may account for the recently observed non-thermal emission from several anomalous X-ray pulsars and soft-gamma repeaters.