Compound lensing: Einstein zig-zags and high-multiplicity lensed images

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Compound strong gravitational lensing is a rare phenomenon, but a handful of such lensed systems are likely to be discovered in forthcoming surveys. In this work, we use a double singular isothermal sphere lens model to analytically understand how the properties of the system impact image multiplicity for the final source. We find that up to six images of a background source can form, but only if the second lens is multiply imaged by the first and the Einstein radius of the second lens is comparable to, but does not exceed that of the first. We then build a model of compound lensing masses in the Universe, using singular isothermal ellipsoid (SIE) lenses, and assess how the optical depth for multiple imaging by a galaxy–galaxy compound lens varies with source redshift. For a source redshift of 4, we find optical depths of 6 × 10−6 for multiple imaging and 5 × 10−8 for multiplicity of 6 or greater. We find that extreme magnifications are possible, with magnifications of 100 or more for 6 × 10−9 of z = 10 sources with 0.1 kpc radii. We show some of the image configurations that can be generated by compound lenses, and demonstrate that they are qualitatively different to those generated by single-plane lenses; dedicated compound lens finders will be necessary if these systems are to be discovered in forthcoming surveys.
Original languageEnglish
Pages (from-to)2210-2220
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date30 Dec 2015
Publication statusPublished - 21 Feb 2016


  • gravitational lensing: strong


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