Caustic Crossing Events within Galaxy Clusters

Abstract: Galaxy clusters are the largest gravitational lenses that can make multiple images of, and substantially magnify, sources at cosmological distances. Individual stars in distant lensed galaxies can be detected as they cross the caustics of lensing clusters. Motivated by the recent discovery of such a system, I will describe the observational characteristics of these events, starting with the characteristic light curves for smooth cluster lenses. I will then show how even a tiny mass fraction of microlenses inevitably disrupts the smooth cluster caustic into a network of corrugated micro-caustics, and produces light curves with numerous peaks over the timescales of typical observations. Caustic crossing events are exquisitely sensitive to granularity in the mass distribution in the outskirts of galaxy clusters, and are thus powerful probes of the population of intracluster stars, and the nature of dark matter.