Pulsar wind nebulae in evolved supernova remnants

For pulsars similar to the one in the Crab Nebula, most of the energy input to the surrounding wind nebula occurs on a timescale less than or similar to 10(3) yr; during this time, the nebula expands into freely expanding supernova ejecta. On a timescale similar to 10(4) yr, the interaction of the supernova with the surrounding medium drives a reverse shock front toward the center of the remnant, where it crushes the pulsar wind nebula (PWN). We have carried out one- and two-dimensional, two-fluid simulations of the crushing and reexpansion phases of a PWN. We show that (1) these phases are subject to Rayleigh-Taylor instabilities that result in the mixing of thermal and nonthermal fluids, and (2) asymmetries in the surrounding interstellar medium give rise to asymmetries in the position of the PWN relative to the pulsar and explosion site. These effects are expected to be observable in the radio emission from evolved PWN because of the long lifetimes of radio-emitting electrons. The scenario can explain the chaotic and asymmetric appearance of the Vela X PWN relative to the Vela pulsar without recourse to a directed flow from the vicinity of the pulsar. The displacement of the radio nebulae in G327.1-1.1, MSH 15-56 (G326.3-1.8), G0.9+0.1, and W44 relative to the X-ray nebulae may be due to this mechanism. On timescales much greater than the nebular crushing time, the initial PWN may be mixed with thermal gas and become unobservable, so that even the radio emission is dominated by recently injected particles.