Strong gravitational lensing

Strong gravitational lensing is a gravitational lensing effect that is strong enough to produce multiple images, arcs, or Einstein rings. Generally, for strong lensing to occur, the projected lens mass density must be greater than the critical density, that is $\Sigma _{cr}$ . For point-like background sources, there will be multiple images; for extended background emissions, there can be arcs or rings. Topologically, multiple image production is governed by the odd number theorem.^[1]

Strong lensing was predicted by Albert Einstein's general theory of relativity and observationally discovered by Dennis Walsh, Bob Carswell, and Ray Weymann in 1979.^[2] They determined that the Twin Quasar Q0957+561A comprises two images of the same object.

^ Mediavilla, Evencio (2016). Astrophysical Applications of Gravitational Lensing. Cambridge University Press. ISBN 978-1-107-07854-3.
^ Bernstein, G. M.; Tyson, J. A.; Kochanek, C. S. (1993). "A large arc in the gravitational lens system 0957 + 561". The Astronomical Journal. 105: 816. Bibcode:1993AJ....105..816B. doi:10.1086/116474. ISSN 0004-6256.

[Mediavilla_2016_p.-1] Mediavilla, Evencio (2016). Astrophysical Applications of Gravitational Lensing. Cambridge University Press. ISBN 978-1-107-07854-3.

[BernsteinTyson1993-2] Bernstein, G. M.; Tyson, J. A.; Kochanek, C. S. (1993). "A large arc in the gravitational lens system 0957 + 561". The Astronomical Journal. 105: 816. Bibcode:1993AJ....105..816B. doi:10.1086/116474. ISSN 0004-6256.

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