TDCOSMO

XVIII. Strong lens model and time-delay predictions for J1721+8842, the first Einstein zigzag lens

¹ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
² Department of Physics and Astronomy, Stony Brook University, Stony Brook NY 11794, USA
³ Kavli Institute for Particle Astrophysics & Cosmology, P.O. Box 2450 Stanford University, Stanford, CA 94305, USA
⁴ STAR Institute, Quartier Agora – Allée du six Août, 19c B-4000 Liège, Belgium
⁵ Technical University of Munich, TUM School of Natural Sciences, Department of Physics, James-Franck-Str 1, 85748 Garching, Germany
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁷ Department of Astronomy & Astrophysics, The University of Chicago, Chicago IL 60637, USA
⁸ Kavli Institute for Cosmological Physics, University of Chicago, Chicago IL 60637, USA
⁹ The Oskar Klein Centre, Department of Physics Stockholm University, SE – 106 91 Stockholm, Sweden
¹⁰ Institute of Physics, Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix, Switzerland
¹¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹² Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès, 1, E-08028 Barcelona, Spain
¹³ ICREA, Pg. Lluís Companys 23, Barcelona E-08010, Spain
¹⁴ Center for Astronomy, Space Science and Astrophysics, Independent University, Bangladesh, Dhaka 1229, Bangladesh

^⋆ Corresponding author: thomas@astro.ucla.edu

Received: 14 March 2024
Accepted: 16 June 2025

Abstract

We present lens models for J1721+8842, the first-ever discovered galaxy-scale strong lens in an Einstein zigzag configuration. The model consists of four separate lensed galaxies, with the primary source hosting a quasar, lensed into six images by two deflectors at redshifts z₁ = 0.184 and z₂ = 1.885. The configuration of three lensed sources and the lensed light of the deflector at redshift z₂ = 1.885 tightly constrain the mass profile of the primary lensing galaxy. Using two standard descriptions for the main perturber’s mass distribution – a total power-law profile and a composite dark and stellar mass – the inferred convergence around the location of the lensed images is in excellent agreement. While the strong lensing data alone does not significantly favor either of our profile assumptions for the main deflector’s mass distribution, we show that a central stellar velocity dispersion measurement can distinguish or validate them. Using a standard ΛCDM cosmology with H₀ = 70 km s⁻¹ Mpc⁻¹, we present time-delay predictions between the lensed quasar images for both models at the percent level modulo a multiplane mass sheet transform. Our models are the first step toward constraining the time-delay distance ratios for J1721+8842, and thus also H₀, independent of other methods. In order to achieve an H₀ measurement, our models need to be combined in a multiplane lensing analysis with the stellar velocity dispersion for the deflectors, the line-of-sight convergence, and the observed time delays. Owing to its extraordinary configuration, this is an extremely promising system for a high-precision determination of H₀.