Anastrophysical jet is anastronomical phenomenon whereionised matter is expelled at high velocity from an astronomical object, in a pair of narrow streams aligned with the object'saxis of rotation.[1] When the matter in the beam approaches thespeed of light, astrophysical jets becomerelativistic jets as they show effects fromspecial relativity.
Astrophysical jets are associated with many types ofhigh-energy astronomical sources, such asblack holes,neutron stars andpulsars. Their causes are not yet fully understood, but they are believed to arise from dynamic interactions withinaccretion disks. One explanation is that as an accretion disk spins, it generates a rotating, tangledmagnetic field which concentrates material from the disk into the jets and then drives it away from the central object.[2] Jets may also be influenced by ageneral relativity effect known asframe-dragging.[3]
Relativistic jets are beams of ionised matter accelerated close to the speed of light. Most have been observationally associated with central black holes of someactive galaxies,radio galaxies orquasars, and also by galacticstellar black holes,neutron stars orpulsars. Beam lengths may extend between several thousand,[6] hundreds of thousands[7] or millions of parsecs.[2] Jet velocities when approaching the speed of light show significant effects of thespecial theory of relativity; for example,relativistic beaming that changes the apparent beam brightness.[8]
Massive central black holes in galaxies have the most powerful jets, but their structure and behaviours are similar to those of smaller galacticneutron stars and black holes. These SMBH systems are often calledmicroquasars and show a large range of velocities.SS 433 jet, for example, has a mean velocity of 0.26c.[9] Relativistic jet formation may also explain observed gamma-ray bursts, which have the most relativistic jets known, beingultrarelativistic.[10]
Mechanisms behind the composition of jets remain uncertain,[11] though some studies favour models where jets are composed of an electrically neutral mixture ofnuclei,electrons, andpositrons, while others are consistent with jets composed of positron–electron plasma.[12][13][14] 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.
Because of the enormous amount of energy needed to launch a relativistic jet, some jets are possibly powered by spinningblack holes. However, the frequency of high-energy astrophysical sources with jets suggests combinations of different mechanisms indirectly identified with the energy within the associated accretion disk and X-ray emissions from the generating source. Two early theories have been used to explain how energy can be transferred from a black hole into an astrophysical jet:
Blandford–Znajek process.[15] This theory explains the extraction of energy from magnetic fields around an accretion disk, which are dragged and twisted by the spin of the black hole. Relativistic material is then feasibly launched by the tightening of the field lines.
The pulsar IGR J11014-6103 with supernova remnant origin, nebula and jet
Jets may also be observed from spinning neutron stars. An example is pulsarIGR J11014-6103, which has the largest jet so far observed in theMilky Way, and whose velocity is estimated at 80% the speed of light (0.8c). X-ray observations have been obtained, but there is no detected radio signature nor accretion disk.[19][20] Initially, this pulsar was presumed to be rapidly spinning, but later measurements indicate the spin rate is only 15.9 Hz.[21][22] Such a slow spin rate and lack of accretion material suggest the jet is neither rotation nor accretion powered, though it appears aligned with the pulsar rotation axis and perpendicular to the pulsar's true motion.
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