An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.
The formation and powering of astrophysical jets are highly complex phenomena that are associated with many types of high-energy astronomical sources. They likely arise from dynamic interactions within accretion disks, whose active processes are commonly connected with compact central objects such as black holes, neutron stars or pulsars. One explanation is that tangled magnetic fields are organised to aim two diametrically opposing beams away from the central source by angles only several degrees wide (c. > 1%). Jets may also be influenced by a general relativity effect known as frame-dragging.
Most of the largest and most active jets are created by supermassive black holes (SMBH) in the centre of active galaxies such as quasars and radio galaxies or within galaxy clusters. Such jets can exceed millions of parsecs in length. Other astronomical objects that contain jets include cataclysmic variable stars, X-ray binaries and gamma-ray bursts (GRB). Others are associated with star forming regions including T Tauri stars and Herbig–Haro objects, which are caused by the interaction of jets with the interstellar medium. Bipolar outflows or jets may also be associated with protostars, or with evolved post-AGB stars, planetary nebulae and bipolar nebulae.
Relativistic jets are beams of ionised matter accelerated close to the speed of light. Most have been observationally associated with central black holes of some active galaxies, radio galaxies or quasars, and also by galactic stellar black holes, neutron stars or pulsars. Beam lengths may extend between several thousand, hundreds of thousands or millions of parsecs. Jet velocities when approaching the speed of light show significant effects of the special theory of relativity; for example, relativistic beaming that changes the apparent beam brightness (see the 'one-sided' jets below).
Massive central black holes in galaxies have the most powerful jets, but their structure and behaviours are similar to those of smaller galactic neutron stars and black holes. These SMBH systems are often called microquasars and show a large range of velocities. SS433 jet, for example, has a velocity of 0.23c. Relativistic jet formation may also explain observed gamma-ray bursts. Notably, even weaker and less relativistic jets may be associated with many binary systems.
Mechanisms behind the composition of jets remain uncertain, though some studies favour models where jets are composed of an electrically neutral mixture of nuclei, electrons, and positrons, while others are consistent with jets composed of positron–electron plasma. 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 spinning black holes. However, the frequency of high-energy astrophysical sources with jets suggest combination 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:
Jets may also be observed from spinning neutron stars. An example is pulsar IGR J11014-6103, which has the largest jet so far observed in the Milky Way Galaxy 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. Initially, this pulsar was presumed to be rapidly spinning but later measurements indicate the spin rate is only 15.9 Hz. 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.
An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.Bipolar outflow
A bipolar outflow comprises two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of bipolar nebulae).Borderlescott
Borderlescott (foaled 12 April 2002) is a British Thoroughbred racehorse. A specialist sprinter noted for his consistency and durability he raced 85 times on 25 different tracks in twelve seasons between 2004 and 2015. He won fourteen races and was placed second or third on thirty occasions. In his early career the gelding showed promising form, winning one minor race as a juvenile in 2004 and four handicap races in the following year. In 2006 he recorded his first major success when he won the Stewards' Cup. He failed to win in 2007 but emerged as a top-class sprinter in the following year when his wins included the Nunthorpe Stakes. He won the Nunthorpe Stakes again in 2009 and added a win in the King George Stakes in 2010. He won the Beverley Bullet Sprint Stakes in 2012 before being retired at the end of the year. He came out of retirement in 2013 and raced nineteen times without success before being retired again in 2015.Cecil Frail Stakes
The Cecil Frail Stakes is a Listed flat horse race in Great Britain open to mares and fillies aged three years or older.
It is run at Haydock Park over a distance of 6 furlongs (1,206 metres), and it is scheduled to take place each year in May.
The race was first run in 1999. Until 1996 the name Cecil Frail was given to a rated handicap at the same course.Firth of Clyde Stakes
The Firth of Clyde Stakes is a Group 3 flat horse race in Great Britain open to two-year-old fillies. It is run at Ayr over a distance of 6 furlongs (1,207 metres), and it is scheduled to take place each year in September.
The event is named after the Firth of Clyde, an area of water off the coast of Ayr. For a period it held Listed status, and it was promoted to Group 3 level in 2004. It is now the only Group race in Scotland.
The Firth of Clyde Stakes is held during the three-day Ayr Gold Cup Festival (previously known as the Western Meeting). It is currently run on the final day, the same day as the Ayr Gold Cup.Flying Five Stakes
The Flying Five Stakes is a Group 1 flat horse race in Ireland open to thoroughbreds aged three years or older. It is run at the Curragh over a distance of 5 furlongs (1,006 metres), and it is scheduled to take place each year in September during Irish Champions Weekend.Jet (fluid)
A jet is a stream of fluid that is projected into a surrounding medium, usually from some kind of a nozzle, aperture or orifice. Jets can travel long distances without dissipating.
Jet fluid has higher momentum compared to the surrounding fluid medium. In the case that the surrounding medium is assumed to be made up of the same fluid as the jet, and this fluid has a viscosity, the surrounding fluid is carried along with the jet in a process called entrainment.Some animals, notably cephalopods, use a jet to propel themselves in water.King George Stakes
The King George Stakes is a Group 2 flat horse race in Great Britain open to horses aged three years or older. It is run at Goodwood over a distance of 5 furlongs (1,006 metres), and it is scheduled to take place each year in late July or early August.List of Vanderbilt University people
This is a list of notable current and former faculty members, alumni, and non-graduating attendees of Vanderbilt University in Nashville, Tennessee.
Unless otherwise noted, attendees listed graduated with bachelor's degrees. Names with an asterisk (*) graduated from Peabody College prior to its merger with Vanderbilt.List of unsolved problems in physics
Some of the major unsolved problems in physics are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon or experimental result. The others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail.
There are still some deficiencies in the Standard Model of physics, such as the origin of mass, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy. Another problem lies within the mathematical framework of the Standard Model itself—the Standard Model is inconsistent with that of general relativity, to the point that one or both theories break down under certain conditions (for example within known spacetime singularities like the Big Bang and the centers of black holes beyond the event horizon).Marek Sikora (astronomer)
Marek Sikora is a Polish astronomer.Habilitation of astrophysics received in 1990 from University of Warsaw. Received the title of professor in 1999. He currently works as a professor in the Centrum Astronomiczne im. Mikołaja Kopernika PAN, Polish Academy of Sciences in Warsaw. He is interested mainly high energy astrophysics, Astrophysical jet, the nuclei of active galaxies, and sources of cosmic radiation.Prohibit (horse)
Prohibit (foaled 14 February 2005) is a retired British Thoroughbred racehorse who excelled over sprint distances, producing most of his best performances over five furlongs. In his first three seasons he was trained by John Gosden and showed useful form, winning three minor races but appearing to be some way short of top class. After being sold and transferred to the stable of Robert Cowell he showed improved form, winning the sprint race at the 2010 Shergar Cup and winning a strongly contested edition of the Scarbrough Stakes. He reached his peak as a six-year-old in 2011 when he won a handicap race in Dubai, the Group One King's Stand Stakes at Royal Ascot and the Prix du Petit Couvert in France as well as running prominently in several other major sprints including the Prix de Saint-Georges, Temple Stakes, Prix du Gros Chene and Nunthorpe Stakes. He remained in training for three more seasons but never won again and was retired in 2014 with a record of nine wins from fifty-nine starts.Relativistic beaming
Relativistic beaming (also known as Doppler beaming, Doppler boosting, or the headlight effect) is the process by which relativistic effects modify the apparent luminosity of emitting matter that is moving at speeds close to the speed of light. In an astronomical context, relativistic beaming commonly occurs in two oppositely-directed relativistic jets of plasma that originate from a central compact object that is accreting matter. Accreting compact objects and relativistic jets are invoked to explain the following observed phenomena: x-ray binaries, gamma-ray bursts, and, on a much larger scale, active galactic nuclei (AGN). (Quasars are also associated with an accreting compact object, but are thought to be merely a particular variety of AGN.)
Beaming (short for relativistic beaming) affects the apparent brightness of a moving object just as a lighthouse affects the appearance of its light source: the light source appears dim or unseen to a ship except when the rotating lighthouse beacon is directed towards a ship, where it then appears very bright. This so-called lighthouse effect illustrates how important the direction of motion (relative to the observer) is in relativistic beaming: consider a blob of gas emitting electromagnetic radiation that is moving relative to the observer. If the gas is moving towards the observer then it will be brighter than if it were at rest, but if the gas isn't moving towards the observer it may (in some cases) appear much fainter than if it were at rest. The importance of this effect in astronomy is illustrated by comparing the AGN jets detected in the galaxy M87 and 3C31 (see figures on the right). The twin jets in M87 show how beaming affects their appearance when one jet moves almost directly towards Earth and the other jet moves in the opposite direction; while M87's jet moving towards Earth is clearly visible to telescopes (the long and thin blue-ish feature in the top image) and is many times brighter due to beaming, M87's other jet is moving away from us and is, due to beaming, so much fainter than the jet directed towards us that it is rendered invisible. 3C31 is different from M87 because both jets (labeled in the figure directly below M87's image) are directed at roughly right angles to our line of sight and are therefore subject to the same amount of beaming. Thus, unlike the case of M87, both of 3C31's jets are visible. The jet displayed on the upper part of the image of 3C31 is actually pointing slightly more in Earth's direction than the other jet and is therefore the brighter of the two.Relativistically moving objects are beamed due to a variety of physical effects. Light aberration causes most of the photons to be emitted along the object's direction of motion. The Doppler effect changes the energy of the photons by red- or blueshifting them. Finally, time intervals as measured by clocks moving alongside the emitting object are different from those measured by an observer on Earth due to time dilation and photon arrival time effects. How all of these effects modify the brightness, or apparent luminosity, of a moving object is determined by the equation describing the relativistic Doppler effect (which is why relativistic beaming is also known as Doppler beaming).Synchrotron radiation
Synchrotron radiation (also known as magnetobremsstrahlung radiation) is the electromagnetic radiation emitted when charged particles are accelerated radially, i.e., when they are subject to an acceleration perpendicular to their velocity (a ⊥ v). It is produced, for example, in synchrotrons using bending magnets, undulators and/or wigglers. If the particle is non-relativistic, then the emission is called cyclotron emission. If, on the other hand, the particles are relativistic, sometimes referred to as ultrarelativistic, the emission is called synchrotron emission. Synchrotron radiation may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields. The radiation produced in this way has a characteristic polarization and the frequencies generated can range over the entire electromagnetic spectrum which is also called continuum radiation.Tidal disruption event
A tidal disruption event (also known as a tidal disruption flare) is an astronomical phenomenon that occurs when a star gets sufficiently close to a supermassive black hole's event horizon and is pulled apart by the black hole's tidal forces, experiencing spaghettification.World Trophy
The World Trophy is a Group 3 flat horse race in Great Britain open to horses aged three years or older. It is run at Newbury over a distance of 5 furlongs and 34 yards (1,037 metres), and it is scheduled to take place each year in September.