2021MNRAS.501.5538D

We study light variability of gravitationally magnified high-redshift star clusters induced by a foreground population of microlenses. This arises as the incoherent superposition of light variations from many source stars traversing the random magnification pattern on the source plane. The light curve resembles a scale-invariant, Gaussian process on time-scales of years to decades, while exhibits rapid and frequent micro-caustic crossing flares of larger amplitudes on time-scales of days to months. For a concrete example, we study a young Lyman-continuum-leaking star cluster in the Sunburst Arc at z = 2.37. We show that one magnified image happens to be intervened by a foreground galaxy, and hence should exhibit a variable flux at the 1-2 per cent level, which is measurable in space with ∼1-3 ks exposures on the Hubble Space Telescope and more easily with the James Webb Space Telescope, or even from the ground using a ∼4-m telescope without adaptive optics. Detailed measurement of this variability can help determine the absolute macro magnification and hence the intrinsic mass and length scales of the star cluster, test synthetic stellar population models, and probe multiplicity of massive stars. Furthermore, monitoring the other lensed images of the star cluster, which are free from significant intervention by foreground microlenses, can allow us to probe planetary to stellar mass compact objects constituting as little as a few per cent of the dark matter. Given the typical surface density of intracluster stars, we expect this phenomenon to be relevant for other extragalactic star clusters lensed by galaxy clusters.