Particle acceleration in colliding flows: Binary star winds and other double-shock structures

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, preenergized 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}≈u_{2}≈c, such particles start bouncing between the shocks at a fixed angle ≈35.3^{∘} to the shock surface. Their drift along the shock fronts is slow, V_{d}∼|u_{2}-u_{1}|≪c, so that it will take N∼Lc/|u_{2}-u_{1}|d≫1 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 tenfold 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.