TheVela Pulsar (centre) and its surrounding pulsar wind nebulaThe innerCrab Nebula. Central part shows the pulsar wind nebula, with the red star in the centre being theCrab Pulsar. Image combines optical data fromHubble (in red) andX-ray data fromChandra (in blue).
Apulsar wind nebula (PWN, pluralPWNe), sometimes called aplerion (derived from the Greek "πλήρης",pleres, meaning "full"),[1] is a type ofnebula sometimes found inside the shell of asupernova remnant (SNR), powered by winds generated by a centralpulsar. These nebulae were proposed as a class in 1976 as enhancements atradio wavelengths inside supernova remnants.[1] They have since been found to beinfrared, optical, millimetre,X-ray[2] andgamma ray sources.[3][4]
Pulsar wind nebulae evolve through various phases.[2][5] New pulsar wind nebulae appear soon after a pulsar's creation, and typically sit inside asupernova remnant, for example theCrab Nebula,[6] or the nebula within the largeVela Supernova Remnant.[7] As the pulsar wind nebula ages, the supernova remnant dissipates and disappears. Over time, pulsar wind nebulae may becomebow-shock nebulae surrounding millisecond or slowly rotating pulsars.[8]
Pulsar winds are composed of charged particles (plasma) accelerated torelativistic speeds by the rapidly rotating, powerfulmagnetic fields above 1teragauss (100 millionteslas) that are generated by the spinning pulsar. The pulsar wind often streams into the surrounding interstellar medium, creating a standingshock 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 highlypolarizedflux and a flatspectral 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).[5]
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.[4]
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