Time-delay quasars: Scales and orders of magnitudes

Abstract

We can think of a lensed quasar as taking the Hubble time, shrinking it by $\sim 10^{-11}$, and then presenting the result to us as a time delay; the shrinking factor is of the order of fractional sky-area that the lens occupies. This cute fact is a straightforward consequence of lensing theory, and enables a simple rescaling of time delays. Observed time delays have a 40-fold range, but after rescaling the range reduces to 5-fold. The latter range depends on details of the lens and lensing configuration – for example, quads have systematically shorter rescaled time delays than doubles – and is as expected from a simple model. The hypothesis that observed time-delay lenses all come from a generalized-isothermal family can be ruled out. But there is no indication of drastically different populations either.