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Relativistic beaming 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 x-ray binaries, gamma-ray bursts, and, on a much larger scale, active galactic nuclei.
Beaming affects the apparent brightness of a moving object in the same way that the direction at which a ship views the light coming from a lighthouse affects the perceived brightness of that light: the light appears dim or unseen to the ship except when the rotating beacon is directed towards it, when it then appears bright. This so-called lighthouse effect illustrates the importance of the direction of motion relative to the observer. Consider a cloud of gas moving relative to the observer and emitting electromagnetic radiation. If the gas is moving towards the observer, it will be brighter than if it were at rest, but if the gas is moving away, it will appear fainter. The magnitude of the effect is illustrated by the AGN jets of the galaxies M87 and 3C 31. M87 has twin jets aimed almost directly towards and away from Earth; the jet moving towards Earth is clearly visible , while the other jet is so much fainter it is not visible. In 3C 31, both jets are at roughly right angles to our line of sight, and thus, both are visible. The upper jet actually points slightly more in Earth's direction and is therefore brighter.
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 blue-shifting 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.