Strong gravitational lensing is a gravitational lensing effect that is strong enough to produce multiple images, arcs, or even Einstein ring

Einstein ring

In observational astronomy an Einstein ring is the deformation of the light from a source into a ring through gravitational lensing of the source's light by an object with an extremely large mass . This occurs when the source, lens and observer are all aligned...

s. Generally, the strong lensing effect requires the projected lens mass density greater than the critical density . For point-like background sources, there will be multiple images; for extended background emissions, there can be arcs or rings. Topologically, the multiple image production is governed by Odd number theorem

Odd number theorem

The odd number theorem is a theorem in strong gravitational lensing which comes directly from differential topology. It says that the number of multiple images produced by a bounded transparent lens must be odd....

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Galaxy lensing

The foreground lens is a galaxy. When the background source is a quasar or resolved jet, the strong lensed images are usually point-like multiple images; When the background source is a galaxy or extended jet emission, the strong lensed images can be arcs or rings.

Cluster lensing

The foreground lens is a galaxy cluster. In this case, the lens is usually powerful enough to produce noticeable both strong lensing (multiple images, arcs or rings) and weak lensing effects (ellipticity distortions).

Mass profiles (DC problems)

Since gravitational lensing is an effect only depending on gravitational potential, it can be used to constrain the mass model of lenses. With the constraints from multiple images or arcs, a proposed mass model can be optimised to fit to the observables. The subgalactic structures currently interests lensing astronomers are the central mass distribution and dark matter halos.

Time delays (AC problems)

Since the light rays go through different paths to produce multiple images, they will get delayed by local potentials along the light paths. The time delay differences from different images can be determined by the mass model and the cosmological model. Thus, with observed time delays and constrained mass model, the cosmological constant like Hubble constant can be inferred.

External links

• CLASS survey database