Interpreting the candidate Galactic microlensing events

Abstract

Four microlensing collaborations are presently searching for compact matter in the Galaxy and all have detected possible candidates. Using the detection efficiencies recently published by the MACHO and OGLE collaborations, we present Monte-Carlo calculations of the expected optical depth, rates and timescales, along the LMC and Galactic bulge lines of sight, for dark matter in a four-component `standard Galaxy&#39; model with a spherically-symmetric halo and spheroid. Using the typically observed event durations we show that, whilst the halo fraction comprised of compact matter is likely to be $f_{h} < 0.4$, a `no halo compact matter&#39; hypothesis is ruled out at greater than the 80\% confidence level, unless the LMC itself has a substantial halo of such objects. On the basis of the timescales observed by OGLE towards the bulge we find the rate predicted by the model to be in good agreement with the number of OGLE detections. We compute lens mass probability distributions for the various components and compare these estimates with current observational and theoretical constraints on the mass scale of baryonic dark matter. We assess the uniformity of the amplification distributions for the published EROS, MACHO and OGLE events and find that they are quite consistent with the microlensing hypothesis, although the OGLE candidate selection criteria mean that its data are particularly sensitive to photometric selection effects. The EROS team has recently placed strong limits on the density contribution of very low mass halo objects from their short timescale CCD search. On the basis of a recent study of the flux amplification of finite-size sources by Simmons, Newsam \& Willis \shortcite{simm95} we suggest that EROS may have detected up to 5 low-amplification events due to halo lenses with mass $m\sim 10^{-7}~\sm$.