Gravitational microlensing by clustered machos

It has been proposed that the MACHOs in our Galaxy could be clumped in globular cluster-like associations or RAMBOs (robust associations of massive baryonic objects) (Moore & Silk). Here we investigate the effect such clustering has on the microlensing of stars in the Large Magellanic Cloud. We find that the lensing in a 1 square degree field could be dominated by just a few clusters. As a result the lensing properties vary widely depending on the position and velocity of those clusters which happen to lie between us and the LMC. Moreover, we find a large variance in timescale distributions that suggests that the small-number statistics could easily be dominated by events in the tails of the unclustered distribution (e.g., by long periods). We compare our results with the MACHO collaboration data and find that a "standard" halo made entirely of MACHOs is not strongly disfavored if the clusters have masses of 106 M. For less massive clusters such a halo is not as likely. For 104 M clusters the microlensing statistics are essentially unchanged from the unclustered case. It may be possible to detect very massive clusters from the distribution of events in timescale and space. We provide some examples of timescale distributions. © 1996. The American Astronomical Society. All rights reserved.