Relativistic beaming

Only a single jet is visible in M87.
Two jets are visible in 3C 31.

In physics, 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 matter that is moving at speeds close to the speed of light. Relativistic beaming causes matter moving towards an observer—for example, a telescope on Earth—to appear brighter than matter moving away, even when there is no intrinsic difference in the luminosities. 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 with relativistic jets explain phenomena such as X-ray binaries, gamma-ray bursts, and, on a much larger scale, active galactic nuclei (AGN; of which quasars are a particular variety).

Beaming affects the apparent brightness of a moving object. If an object emitting electromagnetic radiation moves towards the observer, it will be brighter than if it were at rest, but it 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 (see images at right). M87 has twin jets aimed almost directly towards and away from Earth, so Doppler beaming is extreme; the jet moving towards Earth is clearly visible (the long, thin blueish feature in the top image at right), while the other jet is so much fainter it is not visible. In 3C 31, both jets (labeled in the lower figure at right) are roughly perpendicular to the Earth's line of sight, and thus, both are visible. The upper jet 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 dilation affects the rate at which photons arrive on Earth. 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).

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