Relativistic Beaming

Relativistic beaming (also known as Doppler beaming 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 something just as a lighthouse affects the appearance of its light source: the light source appears dim or unseen to a ship except when the lighthouse is directed towards a ship where it appears very bright. This so-called lighthouse effect illustrates how important the direction of motion (relative to the observer) is in relativistic beaming: if a blob of gas emitting electromagnetic radiation 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. On one hand, 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. On the other hand, 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 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 (which is why relativistic beaming is also known as Doppler beaming).