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 effe ct such clustering has on the microlensing of stars in the Large Magel lanic 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 properti es vary widely depending on the position and velocity of those cluster s which happen to lie between us and the LMC. Moreover, we find a larg e variance in timescale distributions that suggests that the small-num ber statistics could easily be dominated by events in the tails of the unclustered distribution (e.g., by long periods). We compare our resu lts with the MACHO collaboration data and find that a ''standard'' hal o made entirely of MACHOs is not strongly disfavored if the clusters h ave masses of 10(6) M.. For less massive clusters such a halo is not a s likely. For 10(4) M. clusters the microlensing statistics are essent ially unchanged from the unclustered case, It may be possible to detec t very massive clusters from the distribution of events in timescale a nd space. We provide some examples of timescale distributions.