Project Title: The Structure of Pulsar Wind Nebulae

Project Advisor: Dr. Patrick Slane

Background: The general description of a Pulsar Wind Nebula (PWN) like the Crab Nebula seems simple enough: a rapidly rotating, highly magnetic star generates extremely high voltages and accelerates particles to relativistic energies. The result is a magnetized particle wind which inflates a bubble from which synchrotron emission is radiated. This description of a PWN as a calorimeter for the pulsar's spin-down is based directly on measurements of the spin properties and observations of the emission from the radio band through gamma-rays. It provides a beautiful picture of the underlying physics... but it is grossly oversimplified. Recent X-ray observations provide numerous examples of complex features in PWNe. Arcs, rings, jets, and elongated structures are observed in the bright nebulae surrounding a growing number of pulsars, and these structures are telling us how the neutrons stars are transferring their spin energy into the surrounding medium.

Scientific Questions: When current theories for pulsar winds are confronted with the details of recent observations, things get interesting: Particle energies expected from acceleration near the pulsar are far too low to produce the observed X-ray synchrotron emission; where and how does reacceleration occur? Pulsar models yield a small percentage of the available power in particles, yet a variety of measurements appear to require particle-dominated winds; what is the physics of this conversion, and where does it occur? Observations reveal a variety of geometric symmetries in the innermost regions of PWNe accompanied by tangled filaments in their elongated exteriors; how are these formed, and what can they tell us about a pulsar's birth and subsequent evolution?

Scientific Methodology: Our studies with the Chandra and XMM-Newton X-ray Observatories, as well as radio observations with the VLA and ATCA, reveal the complex structure of the wind nebulae produced by young pulsars. High resolution images reveal jets and toroidal structures that allow us to infer the geometry of the pulsar systems. The spectra from these structures constrain models for the energy flow from the pulsar. Identification of the wind termination shock allows us to study the wind as it emerges from the pulsar zone and begins the formation of the PWN. The large scale structure of the nebula provides information on the magnetic field and the presence of filaments formed as the expanding bubble encounters ejecta from the supernova explosion

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