Time-delay quasars: Scales and orders of magnitudes

Astronomy Unit, Queen Mary and Westfield College, University of London, London E1 4NS, UK 
Observatoire astronomique, 11 rue de l'Université, 67000 Strasbourg, France

Corresponding author: p.saha@qmul.ac.uk

Received: 12 August 2003
Accepted: 18 September 2003

Abstract

We can think of a lensed quasar as taking the Hubble time, shrinking it by , 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.