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Caustic crossings in giant arcs with extended dark matter objects

Djuna Croon1,*, Benedict Crossey1,†, Jose Maria Diego2, Bradley J. Kavanagh2,‡, and Jose Maria Palencia2

  • *Contact author: djuna.l.croon@durham.ac.uk
  • †Contact author: benedict.g.crossey@durham.ac.uk
  • ‡Contact author: kavanagh@ifca.unican.es
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Phys. Rev. D 113, 063017 – Published 9 March, 2026

DOI: https://doi.org/10.1103/4g7h-55r2

Abstract

Caustic-crossing stars observed in giant arcs behind galaxy clusters provide a powerful probe of dark matter substructure. While previous work has focused on pointlike lenses such as primordial black holes, we extend this framework to extended dark objects (EDOs), including ultracompact minihalos formed from the collapse of primordial overdensities. We develop an analytic model of microlensing by EDOs embedded in a macrolensing cluster potential and derive the resulting caustics and light curves. Depending on the EDO size relative to the effective Einstein radius, we show that they may generate additional narrow caustics, leading to novel features in the light curve. Applying our framework to the MACS J1149 LS1 “Icarus” event, we constrain EDOs with radii up to 107R⊙. Our results demonstrate that caustic-crossing events complement galactic microlensing searches, as they can probe EDOs with larger physical size. We discuss the implications for current and future observations, which promise to deliver a statistical sample of caustic transients and correspondingly sharper constraints on dark objects.

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Phys. Rev. D 113, 063017– Published 9 March, 2026
  • Received 3 December 2025
  • Accepted 30 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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