Particle acceleration mechanisms at a superluminal shock

Abstract

We consider prospects for cosmic ray acceleration at superluminal shocks and examine in detail the example of the pulsar wind termination shock. We study various models of the shock and acceleration mechanisms. Not only did we modify a transport equation for use in this case but we also wrote a computer program to simulate particle orbits in a simple pulsar wind termination shock model using the parameters suggested by Gallant and Arons (1994). From the study of the particle orbits we found that most particles do not cross the shock more than once. We expect that pitch angle scattering effects should be important, despite their neglect in some previous work, to make a larger fraction of particles recross the shock and gain energy from shock drift acceleration. From our study we found that neither particle orbit nor transport equation methods are sufficient for modeling the acceleration of particles by the shock. However, we find that even when particles gain energy at the shock, they lose their energy outside of the shock if the magnetic field declines with the radius from the pulsar, since particles would drift from the equator to the pole which is in the direction of energy loss. Therefore, the results of our simulations show that whether high energy particles produced by this type of shock could retain that energy on their way out of the pulsar nebula depends strongly on the assumed dependence of B on r in the nebula.