Chapter

Gravitational Lensing: Strong, Weak and Micro

Volume 33 of the series Saas-Fee Advanced Courses pp 453-540

Gravitational Microlensing

  • J. WambsganssAffiliated withZentrum für Astonomie der, Universität Heidelberg (ZAH)

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Gravitational Microlensing can be thought of as a version of strong gravitational lensing in which the image separation is too small to be resolved. Multiple images are formed, but their typical separation – Δθ ≈ 2 θE – is far below the limiting resolution determined by observational constraints. Given the dependence of the Einstein radius on lens mass and geometry, it is clear that microlensing will occur for sufficiently small masses and sufficiently distant lenses and sources. In very general terms, microlensing deals with the lensing effects of compact objects in the mass range 10−6m/M⊙ ≤ 106. This translates into Einstein radii/angular separations of a milli-arcsecond or smaller for the two main distance regimes: “galactic” – lens/source distances of order 10 kpc, and “extragalactic/cosmological” – lens/source distances of order Gpc. Both regimes will be discussed at length in the subsequent sections.