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 for MACHOs in clusters of galaxies focus on monitoring quasars behind nearby systems. This Letter considers the effect of such a compact population on the surface brightness distribution of giant gravitationally lensed arcs. Since the microlensing optical depth is significant in these clusters, the expected fluctuations are substantial and are observable. Focusing on the giant arc seen in Abell 370, we demonstrate that several "extreme" events would be visible in a comparison of Hubble Space Telescope observations at two epochs. Utilizing Next Generation Space Telescope, long-term monitoring should reveal a ubiquitous twinkling of brightness over the surface of the arcs.