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A. Spitkovsky, J. Arons (Departments of Astronomy and Physics, UC Berkeley)
We present results of time-dependent numerical modeling of the internal structure of the collisionless reverse shock terminating the pulsar wind in Crab Nebula. We treat the relativistic wind as composed of ions and electron-positron plasma embedded in a toroidal magnetic field, flowing radially outward from the pulsar in a sector around the rotational equator. Relativistic cyclotron instability of the ion ring downstream of the leading shock in the electron-positron pairs is found to launch outward propagating magnetosonic waves. Due to fresh supply of ions crossing the shock, this time-dependent process achieves a limit-cycle pattern, in which the waves are launched with periodicity on the order of the ion Larmor time. Compressions in magnetic field and pair density associated with these waves as well as their propagation speed reproduce the behavior of features observed in the wisps using the Hubble Space Telescope. By selecting the parameters of the ion orbit to fit the spatial separation of the wisps, we predict a period of time variablitiy that is consistent with the current undersampled data, and set constraints on the number and frequency of observations needed in a future observational campaign to quantitatively test the theory.
Coupled with non-thermal acceleration of the pairs due to their absorption of the waves generated by the ion cyclotron instability, the compressions in the magnetic field and plasma density associated with the waves naturally account for the location and brightness of X-ray features in the Crab, in particular the recently discovered ``inner ring" observed by the Chandra X-ray Observatory.
This work is supported by NASA ATP grant NAG5-3073 and NSF grant AST 9528271