Lens models with density cusps

Lens models appropriate for representing cusped galaxies and clusters are developed. The analogue of the odd‐number theorem for cusped density distributions is given. Density cusps are classified into strong, isothermal or weak, according to their lensing properties. Strong cusps cause multiple imaging for any source position, whereas isothermal and weak cusps give rise to only one image for distant sources. Isothermal cusps always possess a pseudo‐caustic. When the source crosses the pseudo‐caustic, the number of images changes by unity.  Two families of cusped galaxy and cluster models are examined in detail. The double power‐law family has an inner cusp, followed by a transition region and an outer envelope. One member of this family — the isothermal double power‐law model — possesses an exceedingly scarce property, namely the lens equation is exactly solvable for any source position. This means that the magnifications, the time delay and the lensing cross‐sections are all readily available. The model has a three‐dimensional density that is cusped like r −2 at small radii and falls off like r −4 asymptotically. Thus, it provides a very useful representation of the lensing properties of a galaxy or cluster of finite total mass with a flat rotation curve. The second set of models studied is the single power‐law family. These are single density cusps of infinite extent. The properties of the critical curves and caustics and the behaviour of the lenses in the presence of external shear are all discussed in some detail.