Pulsar Wind Nebulae in Evolved Supernova Remnants

For pulsars similar to the one in the Crab Nebula, most of the energy inputto the surrounding wind nebula occurs on a timescale of less than 1000 years;during this time, the nebula expands into freely expanding supernova ejecta. Ona timescale 10,000 years, the interaction of the supernova with the surroundingmedium drives a reverse shock front toward the center of the remnant, where itcrushes the PWN (pulsar wind nebula). One- and two-dimensional, two-fluidsimulations of the crushing and re-expansion phases of a PWN show that (1)these phases are subject to Rayleigh-Taylor instabilities that result in themixing of thermal and nonthermal fluids, and (2) asymmetries in the surroundinginterstellar medium give rise to asymmetries in the position of the PWNrelative to the pulsar and explosion site. These effects are expected to beobservable in the radio emission from evolved PWN because of the long lifetimesof radio emitting electrons. The scenario can explain the chaotic andasymmetric appearance of the Vela X PWN relative to the Vela pulsar withoutrecourse to a directed flow from the vicinity of the pulsar. The displacementof the radio nebulae in G327.1--1.1, MSH15--56 (G326.3--1.8), G0.9+0.1, and W44relative to the X-ray nebulae may be due to this mechanism. On timescales muchgreater than the nebular crushing time, the initial PWN may be mixed withthermal gas and become unobservable, so that even the radio emission isdominated by recently injected particles.Comment: 22 pages, 9 figures; submitted to Ap