Pulsar wind nebula

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**Pulsar wind nebula**
The Crab Nebula, an example of a Pulsar Wind nebula

A pulsar wind nebula (also known as "plerion", derived from Ancient Greek "pleres" meaning "full" - a term coined by Weiler & Panagia (1978)) is a synchrotron nebula powered by the relativistic wind of an energetic pulsar. At the early stage of their evolution, pulsar wind nebulae are often found inside the shells of supernova remnants. A prototypical pulsar wind nebula is the Crab Nebula.

Pulsar wind nebulae are believed to be powered by active rotation-powered pulsars, through a relativistic wind of particles and magnetic fields. The pulsar's wind is decelerated to a sub-relativistic speed across a strong standing shock. The emission from the plerion is seen beyond this radius, and results from synchrotron radiation of the high-energy particles injected by the pulsar in the presence of a strong magnetic field.

Pulsar wind nebulae are generally characterized by the following properties: (1) An increase in brightness towards the center, without a shell-like structure, (2) A highly polarized flux and a flat radio spectral index, α=0-0.3. The index steepens at X-ray energies due to synchrotron and radiation losses and on the average has an X-ray photon index of 1.3-2.3, (3) An X-ray size that is generally smaller than their radio and optical size (due to smaller synchrotron lifetimes of the higher-energy electrons) (Safi-Harb 2004).

Plerions can be powerful probes of a pulsar's interaction with its surroundings — their properties can be used to infer the geometry, energetics, and composition of the pulsar wind, the space velocity of the pulsar itself, and the properties of the ambient medium (Gaensler 2000).