Searching for MACHOs in galaxy clusters

If cluster dark matter is in the form of compact objects, it will introduce fluctuations into the light curves of distant sources. Current searches fo r MACHOs in clusters of galaxies focus on monitoring quasars behind nearby systems. This Letter considers the effect of such a compact population on t he surface brightness distribution of giant gravitationally lensed arcs. Si nce the microlensing optical depth is significant in these clusters, the ex pected fluctuations are substantial and are observable. Focusing on the gia nt are seen in Abell 370, we demonstrate that several "extreme" events woul d be visible in a comparison of Hubble Space Telescope observations at two epochs. Utilizing Next Generation Space Telescope, long-term monitoring sho uld reveal a ubiquitous twinkling of brightness over the surface of the arc s.