Particle Acceleration in Colliding Flows: Binary Star Winds and Other Double-Shock Structures

TL;DR

This paper investigates particle acceleration mechanisms in double-shock structures like colliding stellar winds, highlighting how pre-energized particles can gain significant energy through repeated shock interactions despite certain limitations.

Contribution

It introduces a detailed analysis of particle acceleration in double-shock flows, emphasizing the role of pre-energized particles and their potential for multiple shock crossings leading to high-energy gains.

Findings

Particles can more than ten-fold their energy per bounce between shocks.

Pre-energized particles with large gyroradii can traverse the intershock space multiple times.

Various losses, such as drift and radiative losses, can limit acceleration.

Abstract

A shock wave propagating perpendicularly to an ambient magnetic field accelerates particles considerably faster than in the parallel propagation regime. However, the perpendicular acceleration stops after the shock overruns a circular particle orbit. At the same time, it may continue in flows resulting from supersonically colliding plasmas bound by a pair of perpendicular shocks. Although the double-shock acceleration mechanism, which we consider in detail, is not advantageous for thermal particles, pre-energized particles may avoid the premature end of acceleration. We argue that if their gyroradius exceeds the dominant turbulence scale between the shocks, these particles might traverse the intershock space repeatedly before being carried away by the shocked plasma. Moreover, entering the space between the shocks of similar velocities $u_{1}\approx u_{2}\approx c$, such particles start bouncing between the shocks at a fixed angle $\approx 35.3^{\circ}$ to the shock surface. Their drift along the shock fronts is slow, $V_{d}\sim\left|u_{2}-u_{1}\right|\ll c$, so that it will take $N\sim Lc/\left|u_{2}-u_{1}\right|d\gg1$ bounces before they escape the accelerator (here, $L$ is the size of the shocks and $d$ is the gap between them). Since these particles more than ten-fold their energy per cycle (two consecutive bounces), we invoke other possible losses that can limit the acceleration. They include drifts due to rippled shocks, the nonparallel mutual orientation of the upstream magnetic fields, and radiative losses.