Astrophysical jet

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 associated with the compact central object (such as a black hole or neutron star). 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),^[1] or with evolved post-AGB stars (often in the form of bipolar nebulae).

Unsolved problem in physics: Why do the disks surrounding certain objects, such as the centers of active galaxies, emit jets along their polar axes? (more unsolved problems in physics)

Many stellar objects with accretion disks have jets, although those from super massive black holes are generally the fastest and most active. While it is not known exactly how accretion disks would accelerate jets or produce positron-electron plasma, they are generally thought to generate tangled magnetic fields that cause the jets to accelerate and collimate. The hydrodynamics of a de Laval nozzle may also give a hint to the mechanisms involved.

Relativistic jets

Relativistic jet. The environment around the AGN where the relativistic plasma is collimated into jets which escape along the pole of the supermassive black hole.

Relativistic jets are very powerful jets^[2] of plasma with speeds close to the speed of light that are emitted by the central black holes of some active galaxies (notably radio galaxies and quasars), stellar black holes, and neutron stars. Their lengths can reach several thousand^[3] or even hundreds of thousands of light years.^[4] If the jet speed is close to the speed of light, the effects of the Special Theory of Relativity are significant; for example, relativistic beaming will change the apparent beam brightness (see the 'one-sided' jets below). The mechanics behind both the creation of the jets^[5][6] and the composition of the jets^[7] are still a matter of much debate in the scientific community. Jet composition might vary; 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.^[8][9]

Elliptical galaxy M87 emitting a relativistic jet, as seen by the Hubble Space Telescope.

Massive galactic central black holes have the most powerful jets. Similar jets on a much smaller scale 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 or greater (that is, speeds over roughly 0.99995c), making them some of the swiftest celestial objects currently known.

Jet composition

One of the best observational approaches to investigate the mechanisms which produce jets is to determine the jet composition at radii where they can be observed directly. Most observations and analysis indicate relativistic jets consist primarily of positron-electron plasma.^[10][11][12]

Trace nuclei swept up in a relativistic positron-electron jet would be expected to have extremely high energy, as these heavier nuclei should attain velocity equal to the positron and electron velocity.

Lab production of 5MeV positron-electron beams allows study of characteristics such as the shock effect of GRBs and how different particles interact with and within relativistic positron-electron beams (for example how beam positrons and electrons combine).^[13]

Rotation as possible 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 energy is transferred from a black hole to a jet.

• Blandford–Znajek process.^[14] This is the most popular theory for the extraction of energy from a central black hole. The magnetic fields around an 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.^[15] 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,^[16] and subsequently shown to be a possible mechanism for the formation of jets.^[17]

Relativistic jets from neutron stars

The pulsar IGR J11014-6103 with supernova remnant origin, nebula and jet.

Jets may also be observed from neutron stars, an example being the pulsar IGR J11014-6103, which produces the largest jet observed in the Milky Way Galaxy. This jet is observed in x-rays and has no detected radio signature.^[18] IGR J11014-6103 has an estimated jet velocity of 0.8c. It is not listed in the latest revision of Accreting Millisecond X-Ray Pulsars (AMXPs)^[19] and no accretion material has been detected.^{[20][21][22][23]} This star was presumed to be rapidly spinning but later measurements indicate the spin rate is only 15.9 Hz.^[24][25] This rather slow spin rate and lack of accretion material suggest this 0.8c positron-electron jet is neither rotation nor accretion powered. In the image the jet, aligned with the pulsar rotation axis, is perpendicular to the pulsar's trajectory and extends out over 37 light years (about 10 times the distance from our sun to the nearest visible star). Interestingly, large clouds of positron-electron plasma are sometimes observed near ordinary (non-runaway) neutron stars that have no jets.^[26]

As IGR J11014-6103 does not have an accretion disk nor an event horizon, its 0.8c jet can not be powered by processes like those discussed in the previous section.

Other images

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  Centaurus A in x-rays showing the relativistic jet

• 

  The M87 jet seen by the Very Large Array in radio frequency (the viewing field is larger and rotated with respect to the above image.)

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  Hubble Legacy Archive Near-UV image of the relativistic jet in 3C 66B

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  Galaxy NGC 3862, an extragalactic jet of material moving at nearly the speed of light can be seen at the three o'clock position.^[1]

1. ^ "Hubble Video Shows Shock Collision Inside Black Hole Jet". 27 May 2015. 

See also

• Accretion disk
• Bipolar outflow
• Blandford–Znajek process
• List of plasma (physics) articles

References

External links

• NASA – Ask an Astrophysicist: Black Hole Bipolar Jets
• SPACE.com – Twisted Physics: How Black Holes Spout Off
• Blandford, Roger; Agol, Eric; Broderick, Avery; Heyl, Jeremy; Koopmans, Leon; Lee, Hee-Won (2001). "Compact Objects and Accretion Disks". arXiv:astro-ph/0107228v1. 

Videos

• Hubble Video Shows Shock Collision inside Black Hole Jet (Article)