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Light echo

Astronomical phenomenon caused by light reflected off surfaces distant from the source

thumb|right|309x309px|Reflected light following path B arrives shortly after the direct flash following path A but before light following path C. B and C have the same [[apparent distance]] from the star as seen from [[Earth]].

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 manifests itself only over astronomical distances.

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 motion. |display-authors=etal

Explanation

The distance traveled from one focus to another, via some point on the ellipse, is the same regardless of the point selected.

Direct light from a stellar outburst (white spot) reaches the observer (path 0) followed by light reflected off particles on progressively wider paraboloids (1–5): the observed disc apparently initially expands faster than light but the illusion is due to light reflecting off different unrelated particles

[ (ANIMATION)] ]] 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 in the immediate vicinity of 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 travelled forward as well as away from the star, it produces the illusion of an echo expanding faster than the speed of light. |access-date=2007-04-17 |url-status=dead |archive-url=https://web.archive.org/web/20061125003637/http://www.space.com/scienceastronomy/light_echo_030326.html |archive-date=2006-11-25

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 of the event in an evenly distributed (spherical) cloud for example will appear to the observer to expand at a rate approaching or faster than the speed of light, because the observer may assume the light from B is actually the light from C.

All reflected light rays that originate from the flash and arrive at Earth together will have traveled the same distance. When the rays of light are reflected, the possible paths between the source and Earth that arrive at the same time correspond to reflections on an ellipsoid, with the origin of the flash and Earth as its two foci (see animation to the right). This ellipsoid naturally expands over time.

Examples

V838 Monocerotis

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. |access-date=2017-05-15

Supernovae

Using light echoes, it is sometimes possible to see the faint reflections of historical supernovae. Astronomers calculate the ellipsoid which has Earth and a supernova remnant at its focal points to locate clouds of dust and gas at its boundary. Identification can be done using laborious comparisons of photos taken months or years apart, and spotting changes in the light rippling across the interstellar medium. By analyzing the spectra of reflected light, 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. The first recorded instance of such an echo was in 1936, but it was not studied in detail.

An example is supernova SN 1987A, the closest supernova in modern times. Its light echoes have aided in mapping the morphology of the immediate vicinity |bibcode-access=free |doi-access=free

Another 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. |access-date=2017-05-15

Light echoes have also 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, |bibcode-access=free |doi-access=free |display-authors=etal |bibcode-access=free |doi-access=free

Yet other examples are supernovae SN 1993J |bibcode-access=free |doi-access=free |bibcode-access=free |doi-access=free

Light echo from the 1838-1858 Great Eruption of Eta Carinae were used to study this supernova imposter. A study from 2012, which used light echo spectra from the Great Eruption, found that the eruption was colder compared to other supernova imposters.

Cepheids

Light echoes from RS Puppis propagate through its reflection nebula.

Light echoes were used to determine the distance to the Cepheid variable RS Puppis to an accuracy of 1%. Pierre Kervella at the European Southern Observatory described this measurement as so far "the most accurate distance to a Cepheid". |access-date=2015-10-18

Nova Persei 1901

In 1939, French astronomer Paul Couderc published a study entitled "Les Auréoles Lumineuses des Novae" (Luminous Haloes of the Novae). |bibcode-access=free |bibcode-access=free

ShaSS 622-073 system

The ShaSS 622-073 system is composed of the larger galaxy ShaSS 073 (seen in yellow in the image on the right) and the smaller galaxy ShaSS 622 (seen in blue) that are at the very beginning of a merger. The bright core of ShaSS 073 has excited with its radiation a region of gas within the disc of ShaSS 622; even though the core has faded over the last 30,000 years, the region still glows brightly as it re-emits the light.

Quasar light and ionisation echoes

HsV is made entirely of gas so hotabout 10,000 degrees Celsiusthat astronomers felt it had to be illuminated by something powerful. |access-date=2016-09-22

Gallery

Eta Carinae light echo.gif|Light echo at Eta Carinae Cassiopeia A infrared echo (north).gif|Light echo at Cassiopeia (north) played back and forth Cassiopeia A infrared echo (south).gif|Light echo at Cassiopeia (south) played back and forth File:Cassiopeia A infrared echo JWST (left).jpg|Detailed view of the Cassiopeia A light echo with JWST NIRCam File:Cassiopeia A infrared echo JWST (right).jpg|Another detailed view of the Cassiopeia A light echo with JWST NIRCam

References

"Anatomy of a light echo".
(Feb 2012). "Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption". Nature.
(2008). "The long-period Galactic Cepheid RS Puppis: I. A geometric distance from its light echoes". Astronomy & Astrophysics.
"An echo of light".

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