Pulsar wind nebula
A pulsar wind nebula (PWN, plural PWNe), sometimes called a plerion (derived from the Greek "πλήρης", pleres, meaning "full"), is a type of nebula found inside the shells of supernova remnants (SNRe) that is powered by pulsar winds generated by its central pulsar. These nebulae were discovered in 1976 as small depressions at radio wavelengths near the centre of supernova remnants. They have since been found to be X-ray emitters and are possibly γ-ray sources.
Evolution of pulsar wind nebulae
Processes creating pulsar wind nebulae are complicated and they evolve through various phases before creating a so-called relic nebula, which is visible as a wind bubble, shell nebula, or as a bow-shock. New plerions appear within the first few thousands of years of a pulsar's creation, and often look like a series of shells inside the supernova remnant, for example the small pulsar wind nebula within the inner region of the Crab Nebula, or the nebula within the large Vela Supernova Remnant and its associated Vela Pulsar.
As the plerion ages, the nebulosity of the supernova remnant dissipates and disappears. Over time, pulsar wind nebulae may change in behaviour and become relic nebulae surrounding millisecond radio pulsars or even older and slower rotating pulsars. Plerions are estimated to last around 15,000 years, after which the shell dissipates as the energies from the pulsar decreases and they are no longer detectable. Importantly, this depends on the rate of energy lost by the pulsar as its spin rate slows, which varies among the known pulsars.
Properties of pulsar wind nebulae
Pulsar winds are composed of charged particles (plasma) accelerated to relativistic speeds by the rapidly rotating, hugely powerful magnetic fields above 1 teragauss (100 million teslas) that are generated by the spinning pulsar. The pulsar wind often streams into the surrounding interstellar medium, creating a standing shock wave called the 'wind termination shock', where matter is decelerated to sub-relativistic speed. Beyond this radius, synchrotron emission increases in the magnetized flow. These processes can switch on and off with many reversals, and this creates the numerous visible shells centred on the pulsar.
Pulsar wind nebulae often show the following properties:
- An increasing brightness towards the center, without a shell-like structure as seen in most other supernova remnants.
- A highly polarized flux and a flat spectral index in the radio band, α=0–0.3. The index steepens at X-ray energies due to synchrotron radiation losses and on the average has an X-ray photon index of 1.3–2.3 (spectral index of 2.3–3.3).
- An X-ray size that is generally smaller than their radio and optical size (due to smaller synchrotron lifetimes of the higher-energy electrons).
- A photon index at TeV gamma-ray energies of ~2.3.
Pulsar wind nebulae can be powerful probes of a pulsar/neutron star's interaction with its surroundings. Their unique 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.
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