Astronomers have found individual “supermagnified” stars far away by using the magnifying power of galaxy clusters. These stars lie very near a lensing caustic — a place in the sky where the lensing magnification becomes very large — and can be boosted by factors of order 1,000 so they become visible at cosmological distances. The new paper explains how the small-scale flickers seen when these stars cross tiny caustics can be used to probe minute lumps or irregularities in the mass of the lensing cluster, including possible dark-matter minihalos.

The authors model how intracluster stars — ordinary stars that float between galaxies in a cluster — break the large-scale caustic into a corrugated band of many micro-caustics. For a typical cluster the authors adopt a surface density of microlenses κ⋆≈0.005, which leads to a band width on the sky of about 0.1 arcseconds and micro-caustic spacings of order 10 microarcseconds. A source star crossing that band will typically meet about 10,000 micro-caustics while crossing the whole band, with an average rate near one micro-caustic crossing per year. The highest magnification reached at a single micro-caustic crossing is reduced compared with a perfectly smooth lens, and for a red supergiant the paper estimates peak magnifications around 1,000.

At a high level, the method is simple to describe. When a star crosses a micro-caustic, the magnification briefly spikes. The detailed shape of that spike — the lightcurve — depends on the star’s apparent size and surface brightness profile. Because the magnification near a caustic grows roughly as the inverse square root of the distance to the caustic, well-sampled photometry of a crossing can effectively resolve the tiny angular disk of the star. The authors show model lightcurves that include limb darkening (the drop of brightness toward the star’s edge) and use those forms to fit the observed event known as Icarus, the first supermagnified star found.