Pulsar wind nebula

A pulsar wind nebula (PWN), sometimes called a plerion is a type of nebula sometimes found inside the shell of a supernova remnant (SNR), powered by winds generated by a central pulsar. These nebulae were proposed as a class in 1976 as enhancements at radio wavelengths inside supernova remnants. They have since been found to be infrared, optical, millimetre, X-ray and gamma ray sources.

Pulsar wind nebulae can be powerful probes of a pulsar/neutron star’s interaction with its surroundings.

Pulsar winds are composed of 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.

Pulsar wind nebulae evolve through various phases. New pulsar wind nebulae appear soon after a pulsar’s creation, and typically sit inside a supernova remnant, for example the Crab Nebula, or the nebula within the large Vela Supernova Remnant. As the pulsar wind nebula ages, the supernova remnant dissipates and disappears. Over time, pulsar wind nebulae may become bow-shock nebulae surrounding millisecond or slowly rotating pulsars.

Vela is a radio, optical, X-ray- and gamma-emitting pulsar associated with the Vela Supernova Remnant in the constellation of Vela. Its parent Type II supernova exploded approximately 11,000–12,300 years ago (and was about 800 light-years away).

Vela is the brightest pulsar (at radio frequencies) in the sky and spins 11.195 times per second (i.e. a period of 89.33 milliseconds—the shortest known at the time of its discovery) and the remnant from the supernova explosion is estimated to be travelling outwards at 1,200 km/s (750 mi/s).

The inner part of the Crab Nebula is dominated by a pulsar wind nebula enveloping the pulsar. Some sources consider the Crab Nebula to be an example of both a pulsar wind nebula as well as a supernova remnant, while others separate the two phenomena based on the different sources of energy production and behaviour.

The pulsar wind often streams into the surrounding interstellar medium, creating a standing shock wave called the ‘wind termination shock’, where the wind decelerates to sub-relativistic speed. Beyond this radius, synchrotron emission increases in the magnetized flow.

Pulsar wind nebulae often show the following properties:

  • An increasing brightness towards the center, without a shell-like structure as seen in 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.

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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