Polar Jet

A polar jet is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. It is usually caused by the dynamic interactions within an accretion disc. When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. The largest polar jets are those seen in active galaxies such as quasars. Other systems which often contain polar jets include cataclysmic variable stars, X-ray binaries and T Tauri stars. Herbig–Haro objects are caused by the interaction of polar 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)

While it is still mostly a mystery to physicists how polar jets are formed and powered, the two most often proposed origins of this power are the central object (such as a black hole), and the accretion disc. Accretion discs around many stellar objects are able to produce jets, although those around a black hole 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 protostar jets are much slower. While it is not known exactly how accretion discs 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 polar jets are produced is to determine the composition of the jets at a radius where they can be directly observed. For example, if a jet originates in the accretion disc, its plasma is likely to have electron-ion composition, whereas if it originates in the black hole it will likely be electron-positron in nature. Also, the plasma emits various forms of radiation such as X-rays and radio waves, which aid diagnosis.