Caustic
Curve in the source plane separating regions of differing image number. Point sources on a caustic are infinitely magnified.Caustic Network
The set of caustics produced by the combined effect of many lenses. Caustic networks arise in microlensing where large numbers of stars must be considered.Chang-Refsdal Lens
A point mass embedded in a uniform convergence and shear field.Convergence
Surface density of lensing object in units of the critical density for lensing. The convergence gives the isotropic gravitational focusing. (See also, Shear.)Critical Curve
Curve in the lens plane that maps to a caustic in the source plane.Deflection Angle
Angle through which a light ray emitted from a light source is deflected by a gravitational lens. The deflection angle is given in terms of the lens potential (mathematically by its gradient).Einstein Radius
Radius of Einstein ring given either in spatial or angular units. It is empiracally known that sources much larger than a given object's Einstein radius will not be magnified by that object. Stellar Einstein radii have typical values of microarcseconds while dark matter substructures are typically on milliarcsecond scales. (See also, Microlensing and Millilensing.)Einstein Ring
Circular image produced when the observer, lens and source are colinear. The term may also be applied to extended sources which can also can be lensed into ring like images. The radius of an Einstein ring is called the Einstein Radius.Fermat Potential
See Time-Delay Surface.Flux Ratio Anomaly
Refers to an observed image flux ratio that does not agree with the ratio predicted by standard macromodels.Galaxy-Galaxy Lensing
Lensing in which both the source and lens are galaxies. This term typically refers to weak lensing, but a number of strong galaxy-galaxy lens systems have been recently discovered. (related papers)Isothermal Models
Models for lens galaxies which yield flat rotation or velocity dispersion curves. In thermodynamics, a distribution of particles with equal velocities is described by a single temperature, whence the term isothermal. Isothermal models are classified by their angular structure and behavior at the center of the mass distribution. Singular models have cusps, i.e. infinite densities at the center, while non-singular models have cores, i.e. constant density within a specified radius. Singular isothermal spheres (SISs) and singular isothermal ellipsoids (SIEs) are most commonly used. External shear is frequently added since many lensing galaxies lie in groups or clusters.Lens Equation
Relation between source position and image positions through the deflection angle.Lens Potential
Projected gravitational potential in dimensionless units. (See also, Deflection Angle)Lensing
The deflection of light by massive objects. In weak lensing only one image is produced. In strong lensing, multiple images, usually two or four, are produced.Macromodel
Description of the lensing galaxy in terms of a simple density profile, e.g., an isothermal sphere. The term does not apply to models that include substructures like dark matter clumps and individual stars.Microlensing
Strong lensing in which image separations are on microarcsecond scales. The term typically applies to lensing of quasar continuum and broad-line emission regions by a foreground galaxy. The sizes of these sources are comparable to stellar Einstein radii and therefore stars must be explicitly included in microlensing calculations. Although resolution on microarcsecond scales is not possible, certain stellar configurations give rise to magnifications that differ substantially from values predicted by a macromodel.Millilensing
Strong lensing in which image separations are on milliarcsecond scales. The term typically applies to lensing of quasar radio lobes/jets by a foreground galaxy. Models for millilensing consist of a macromodel and may also include a clumpy component for the lensing galaxy.Parity
Specifies whether an image is a minimum, maximum, or saddle point of the time-delay surface. A positive parity image typically refers to a minimum. Strictly speaking, the term could also refer to a maximum, although such images tend to be faint and are rarely observed. Negative parity refers to a saddle image.Shear
Second rank tensor that gives the anisotropic light deflection. Produced by tidal forces originating from within the lensing galaxy (internal shear) or from its environment (external shear). One often works in coordinates that are aligned with the shear. In this case, a single number is needed. (See also, Convergence.)Substructure
Small scale structure in the lens galaxy in the form of stars or dark matter clumps. Substructure is typically invoked to explain flux ratio anomalies.Thin Lens Approximation
The assumption that light deflection occurs in a plane perpendicular to the line of sight with its origin coincident with the center of the lens potential.Time Delay
Difference in arrival time for light rays coming from seperate lensed images of the same background source. The time delay consists of a geometric term that accounts for the difference in path length and a gravitational term that depends on the spatial variation of the lens potential. It is possible to use observed time delays to measure either the hubble constant or the slope of the lens potential if one of the two is known.Time-Delay Surface (Fermat Potential)
Mathematical surface that gives the increase in travel time for a light ray passing through a gravitational field. Images form at extrema of the time delay surface.Weak-Lensing
Lensing in which only a single image of a background source is produced. Since the distortion of a given weakly-lensed source is "small," statistically meaningful datasets must be obtained in order to draw conclusions about either the lens or the source.
Last updated on May 27, 2006