Superluminal electromagnetic waves in pulsar winds

Using a combination of analytical methods and Particle-In-Cell (PIC) simulations, we discuss the properties of a coherent, nonlinear, electromagnetic wave propagating in a neutron star wind. For the specific case of the Crab, the canonical flow parameters are such that a circularly polarised, transverse electromagnetic (TEM) wave will undergo a transition from subluminal to superluminal propagation between the inner and outer wind regions, which might involve radiative dissipation. As the superluminal wave encounters the wind’s termination shock, it locally affects the state of the hot, radiative plasma in the shock interior and the downstream medium, possibly with an observable signature. PIC simulations indicate that the wave ultimately decays after some 102−103 plasma skin depths; i.e., over an angular scale of up to 0.05″ for the Crab. To complement our simulations, we solve generalised Rankine–Hugoniot shock jump conditions for a plasma carrying a coherent TEM wave, for a broad range of plasma and wave parameters.