High-energy emission from pulsar binaries

TL;DR

This paper explores how pulsar winds convert electromagnetic energy into high-energy particles in binary systems, proposing a model involving electromagnetic waves and shock interactions that explains observed emissions.

Contribution

It introduces a novel scenario where a pulsar striped wind transforms into a strong electromagnetic wave before the shock, affecting particle acceleration and emission processes.

Findings

Electromagnetic wave conversion efficiency depends on external medium density.

Two regimes identified: wave suppression by reconnection and wave damping in low-density environments.

Binary phase influences shock regime transitions and emission characteristics.

Abstract

Unpulsed, high-energy emission from pulsar binaries can be attributed to the interaction of a pulsar wind with that of a companion star. At the shock between the outflows, particles carried away from the pulsar magnetosphere are accelerated and radiate both in synchrotron and inverse Compton processes. This emission constitutes a significant fraction of the pulsar spin-down luminosity. It is not clear however, how the highly magnetized pulsar wind could convert its mainly electromagnetic energy into the particles with such high efficiency. Here we investigate a scenario in which a pulsar striped wind converts into a strong electromagnetic wave before reaching the shock. This mode can be thought of as a shock precursor that is able to accelerate particles to ultrarelativistic energies at the expense of the electromagnetic energy it carries. Radiation of the particles leads to damping of the wave. The efficiency of this process depends on the physical conditions imposed by the external medium. Two regimes can be distinguished: a high density one, where the EM wave cannot be launched at all and the current sheets in the striped wind are first compressed by an MHD shock and subsequently dissipate by reconnection, and a low density one, where the wind can first convert into an electromagnetic wave in the shock precursor, which then damps and merges into the surroundings. Shocks in binary systems can transit from one regime to another according to binary phase. We discuss possible observational implications for these objects.