Einstein ring

From Wikipedia, the free encyclopedia

In observational astronomy a Chwolson ring or Einstein ring is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational deflection of the source's light by a lens (such as another galaxy, or a black hole). This occurs when the source, lens and observer are all aligned.

Contents 1 Introduction 2 History 2.1 Known Einstein rings 2.2 A simulation 3 References 3.1 Journals 3.2 News 4 See also 5 Further reading

[edit] Introduction

Gravitational lensing is a result from Albert Einstein's theory of General relativity. Instead of light from a source traveling in a straight line (in three dimensions), it is bent by the presence of a massive body, which distorts spacetime. An Einstein Ring is a special case of gravitational lensing, caused by the exact alignment of the source, lens and observer. This results in a symmetry around the lens, causing a ring-like structure.

The size of an Einstein ring is given by the Einstein radius. In radians, it is

,

where

G is the gravitational constant,

M is the mass of the lens,

c is the speed of light,

d_L is the distance to the lens,

d_S is the distance to the source, and

d_LS is the distance between the lens and the source.

[edit] History

The bending of light by a gravitational body was predicted by Einstein in 1912, a few years before the publication of General Relativity in 1916 (see Renn et al. 1997). The ring effect was first mentioned in academic literature by Chwolson in 1924. Albert Einstein remarked upon this effect in 1936, but stated

"Of course, there is no hope of observing this phenomenon directly. First, we shall scarcely ever approach closely enough to such a central line. Second, the angle β will defy the resolving power of our instruments."

In this statement, β is the Einstein Radius and is modernly denoted θ_E (see later). However, Einstein was only considering the chance of observing Einstein rings produced by stars, which is low; however, the chance of observing those produced by larger lenses such as galaxies or black holes is higher since the angular size of an Einstein ring increases with the mass of the lens.

[edit] Known Einstein rings

Hundreds of gravitational lenses are known nowadays. About half a dozen of them are partial Einstein rings with diameters up to an arcsecond, although as either the mass distribution of the lenses are not perfectly axially symmetrical, or the source, lens and observer are not perfectly aligned, we have yet to see a perfect Einstein ring. Most rings have been discovered in the radio range.

Name	Location (RA, dec)	Radius	Arc size	Optical/radio	Discovery
FOR J0332-3357	03h:32m:59s:94, -35°57'51".7, J2000	1".48	Partial, 260°	Radio	Cabanac (2005)

[edit] A simulation

To the right is a simulation depicting a zoom on a Schwarzschild black hole in front of the Milky Way. The first Einstein ring corresponds to the most distorted region of the picture and is clearly depicted by the galactic disc. The zoom then reveals a series of 4 extra rings, increasingly thinner and closer to the black hole shadow. They are easily seen through the multiple images of the galactic disk. Odd rings correspond to points which are behind the black hole (from the observer point of view) and correspond here to the bright yellow region of the galactic disc (close to the galactic center), whereas even rings correspond to images of regions which are behind the observer, which appear bluer since the corresponding part of the galactic disk is dimmer here.

Only black holes can exhibit such multiple rings. The gravitational distortions caused by a star of a galaxy cluster do not allow enough bending of light to produce the extra rings.^{[citation needed]}

[edit] References

[edit] Journals

[edit] News

[edit] See also

• Einstein radius
• Gravitational lens

[edit] Further reading

• Kochanek, C.S.; C.R. Keeton and B.A. McLeod (2001). "The Importance of Einstein Rings". The Astrophysical Journal 547: 50—59.