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A light echo is a physical phenomenon caused by light reflected off surfaces distant from the source, and arriving at the observer with a delay relative to this distance. The phenomenon is analogous to an echo of sound, but due to the much faster speed of light it mostly only manifests itself over astronomical distances.
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For example, a light echo is produced when a sudden flash from a nova is reflected off a cosmic dust cloud, and arrives at the viewer after a longer duration than it otherwise would have taken with a direct path. Because of their geometries, light echoes can produce the illusion of superluminal speeds.
Explanation
Light echoes are produced when the initial flash from a rapidly brightening object such as a nova is reflected off intervening interstellar dust which may or may not be associated with the source of the light. Light from the initial flash arrives at the viewer first, while light reflected from dust or other objects between the source and the viewer begins to arrive shortly afterward. Because this light has only traveled forward as well as away from the star, it produces the illusion of an echo expanding faster than the speed of light.
In the first illustration above, light following path A is emitted from the original source and arrives at the observer first. Light which follows path B is reflected off a part of the gas cloud at a point between the source and the observer, and light following path C is reflected off a part of the gas cloud perpendicular to the direct path. Although light following paths B and C appear to come from the same point in the sky to the observer, B is actually significantly closer. As a result, the echo appears to the observer to expand at a rate faster than the speed of light.
All reflected light that originates from the flash will travel the same distance. When the ray of light is reflected, the possible paths between the source and the Earth correspond to reflections on an ellipsoid, with the origin of the flash and the Earth as its two foci (see animation to the right). This ellipsoid naturally expands over time.
Examples
The variable star V838 Monocerotis experienced a significant outburst in 2002 as observed by the Hubble Space Telescope. The outburst proved surprising to observers when the object appeared to expand at a rate far exceeding the speed of light as it grew from an apparent visual size of 4 to 7 light years in a matter of months.
Light echoes were used to determine the distance to the Cepheid variable RS Puppis to an accuracy of 1%. Pierre Kavella at the European Southern Observatory described this measurement as so far "the most accurate distance to a Cepheid".
Using light echoes, it is sometimes possible to see the faint reflections of historical supernovae. Astronomers calculate the ellipsoid which has the Earth and a supernova remnant at its focal points in order to locate clouds of dust and gas at its boundary. By analyzing the spectra of reflected light from these nebulae, astronomers can discern chemical signatures of supernovae whose light reached Earth long before the invention of the telescope and compare the explosion with its remnants, which may be centuries or millennia old. One example is the SN 1572 supernova observed on Earth in 1572, where in 2008, faint light-echoes were seen on dust in the northern part of the Milky Way. Light echoes can be identified by comparing photos of gas and dust clouds taken months or years apart and spotting changes in the light rippling across the clouds. If the source of the light is unknown, several such observations can be fitted to an ellipsoid to allow astronomers to pinpoint the origin.
Light echoes have been used to study the supernova that produced the supernova remnant Cassiopeia A. The light from Cassiopeia A would have been visible on Earth around 1660, but went unnoticed, probably because dust obscured the direct view. Reflections from different directions allow astronomers to determine if a supernova was asymmetrical and shone more brightly in some directions than in others. The progenitor of Cassiopeia A has been suspected as being asymmetric, and looking at the light echoes of Cassiopeia A allowed for the first detection of supernova asymmetry in 2010.
Light echoes have also been observed in connection with supernovae SN 1993J and SN 1987A, the closest supernova in modern times. The first recorded instance of a light echo was 1936, but it was not studied in detail.
In 1939, French astronomer Paul Couderc published a study entitled "Les Auréoles Lumineuses des Novae" (Luminous Haloes of the Novae). Within this study, Couderc published the derivation of echo locations and time delays in the paraboloid, rather than ellipsoid, approximation of infinite distance. However, in his 1961 study, Y.K. Gulak queried Couderc's theories: "It is shown that there is an essential error in the proof according to which Couderc assumed the possibility of expansion of the bright ring (nebula) around N Per 1901 with a velocity exceeding that of light." He continues: "The comparison of the formulas obtained by the author, with the conclusions and formulas of Couderc, shows that the coincidence of the parallax calculated according to Coudrec's scheme, with parallaxes derived by other methods, could have been accidental."
Quasar light and ionisation echoes
Within the last decade, objects known either as quasar light echoes or quasar ionisation echoes have been investigated. A well studied example of a quasar light echo is the object known as Hanny's Voorwerp (HsV).
HsV is made entirely of gas so hot — about 10,000 Celsius — that astronomers felt it had to be illuminated by something powerful. After several studies of light and ionisation echoes, it is thought they are likely caused by the 'echo' of a previously-active AGN that has shut down. Kevin Schawinski, a co-founder of the website Galaxy Zoo, stated: "We think that in the recent past the galaxy IC 2497 hosted an enormously bright quasar. Because of the vast scale of the galaxy and the Voorwerp, light from that past still lights up the nearby Voorwerp even though the quasar shut down sometime in the past 100,000 years, and the galaxy's black hole itself has gone quiet." Chris Lintott, also a co-founder of Galaxy Zoo stated: "From the point of view of the Voorwerp, the galaxy looks as bright as it would have before the black hole turned off – it's this light echo that has been frozen in time for us to observe." The analysis of HsV in turn has led to the study of objects called Voowerpjes and Green bean galaxies.