An astrophysical jet (hereafter 'jet') is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. While it is still the subject of ongoing research to understand how jets are formed and powered, the two most often proposed origins are dynamic interactions within the accretion disk or a process from the central object (such as a black hole). When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets, because the effects of special relativity become important. The largest jets are those from black holes in active galaxies such as quasars and radio galaxies. Other systems which often contain jets include cataclysmic variable stars, X-ray binaries and T Tauri stars. Herbig–Haro objects are caused by the interaction of jets with the interstellar medium. Bipolar outflows or jets may also be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of bipolar nebulae).
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Accretion disks around many stellar objects are able to produce jets, although those from super massive black holes are the fastest and most active. This is because the speed of the jet is around the same speed as the escape velocity of the central object. This makes the speed of a jet from an accreting black hole near the speed of light, while protostellar jets are much slower. While it is not known exactly how accretion disks manage to produce jets, they are thought to generate tangled magnetic fields that cause the jets to collimate. The hydrodynamics of a de Laval nozzle may also give a hint to the mechanisms involved.
One of the best ways of exploring how jets are produced is to determine the composition of the jets at a radius where they can be directly observed. For example, it has been suggested if a jet originates from the accretion disk, its plasma is likely to have ion-electron composition, whereas if it originates from the black hole or neutron star it will likely be positron-electron in nature. Also, the plasma emits various forms of radiation such as X-rays and radio waves, which aid diagnosis.
Relativistic jets are extremely powerful jets of plasma, with speeds close to the speed of light, that are emitted near the central massive objects of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years. Because the jet speed is close to the speed of light, the effects of the Special Theory of Relativity are important; in particular, relativistic beaming will change the apparent brightness. The mechanics behind both the creation of the jets and the composition of the jets are still a matter of much debate in the scientific community. In terms of composition, some studies favour a model in which the jets are composed of an electrically neutral mixture of nuclei, electrons, and positrons, while others are consistent with a jet primarily of positron-electron plasma.
Similar jets, though on a much smaller scale, can develop from neutron stars and stellar black holes. These systems are often called microquasars. An example is SS433, whose well-observed jet has a velocity of 0.23c, although other microquasars appear to have much higher (but less well measured) jet velocities. Even weaker and less relativistic jets may be associated with many binary systems; the acceleration mechanism for these jets may be similar to the magnetic reconnection processes observed in the Earth's magnetosphere and the solar wind.
The general hypothesis among astrophysicists is that the formation of relativistic jets is the key to explaining the production of gamma-ray bursts. These jets have Lorentz factors of ~100 (that is, speeds of roughly 0.99995c), making them some of the swiftest celestial objects currently known.
IGR J11014-6103 has the largest jet observed in the Milky Way Galaxy. Fortunately this helical jet is not obscured by explosion debris or accretion material. In the image shown (click on image to enlarge) the rapidly spinning neutron star (or pulsar) is the point-like object with a wind tail nebula trailing behind it for about 3 light years. The jet, aligned with the pulsar’s rotation axis, is perpendicular to the pulsar's trajectory and extends out over 37 light years (about 10X the distance from our solar syetem to the nearest star). The estimated jet velocity is 0.8c. This high speed runaway pulsar was created and ejected about 15,000 years ago from the supernova explosion which created supernova remnant SNR MSH 11-16A. Notably this pulsar apparently does not have an accretion disk.     
Rotating black hole as energy source
Because of the enormous amount of energy needed to launch a relativistic jet, some jets are thought to be powered by spinning black holes. There are two well known theories for how the energy is transferred from the black hole to the jet.
- Blandford–Znajek process. This is the most popular theory for the extraction of energy from the central black hole. The magnetic fields around the accretion disk are dragged by the spin of the black hole. The relativistic material is possibly launched by the tightening of the field lines.
- Penrose mechanism. This extracts energy from a rotating black hole by frame dragging. This theory was later proven to be able to extract relativistic particle energy and momentum, and subsequently shown to be a possible mechanism for the formation of jets.
A Hubble Space Telescope survey indicated that relativistic jets may be more likely to form from supermassive black holes resulting from the merger of two galaxies and their galaxy centre's black holes. Not all galaxy mergers create relativistic jets. NASA/ESA Video
Centaurus A in x-rays showing the relativistic jet
Galaxy NGC 3862, an extragalactic jet of material moving at nearly the speed of light can be seen at the three o'clock position.
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|last1=in Authors list (help)
- "Hubble Video Shows Shock Collision Inside Black Hole Jet". 27 May 2015.
- NASA - Ask an Astrophysicist: Black Hole Bipolar Jets
- SPACE.com - Twisted Physics: How Black Holes Spout Off
- Compact Objects and Accretion Disks