Superluminal Waves in Pulsar Winds

The energy lost by a rotation-powered pulsar is carried by a relativistic flow containing a mixture of electromagnetic fields and particles. In the inner regions, this is thought to be a magnetically dominated, cold, electron-positron wind that is well described by the MHD equations. However, beyond a critical radius r _cr, the same particle, energy, and momentum fluxes can be transported by a strong, transverse electromagnetic wave with superluminal phase speed. We analyze the nonlinear dispersion relation of these waves for linear and circular polarization, and find the dependence of r _cr on the mass-loading, magnetization, and luminosity of the flow, as well as on the net magnetic flux. We show that, for most isolated pulsars, the wind lies well outside r _cr and speculate that superluminal modes play an important role in the dissipation of electromagnetic energy into nonthermal particles at the termination shock.