Interpreting the candidate Galactic microlensing events

Four microlensing collaborations are presently searching for compact matterin the Galaxy and all have detected possible candidates. Using the detectionefficiencies recently published by the MACHO and OGLE collaborations, wepresent Monte-Carlo calculations of the expected optical depth, rates andtimescales, along the LMC and Galactic bulge lines of sight, for dark matter ina four-component `standard Galaxy' model with a spherically-symmetric halo andspheroid. Using the typically observed event durations we show that, whilst thehalo fraction comprised of compact matter is likely to be $f_{h} < 0.4$, a `nohalo compact matter' 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 therate predicted by the model to be in good agreement with the number of OGLEdetections. We compute lens mass probability distributions for the variouscomponents and compare these estimates with current observational andtheoretical constraints on the mass scale of baryonic dark matter. We assessthe uniformity of the amplification distributions for the published EROS, MACHOand OGLE events and find that they are quite consistent with the microlensinghypothesis, although the OGLE candidate selection criteria mean that its dataare particularly sensitive to photometric selection effects. The EROS team hasrecently placed strong limits on the density contribution of very low mass haloobjects from their short timescale CCD search. On the basis of a recent studyof the flux amplification of finite-size sources by Simmons, Newsam \& Willis\shortcite{simm95} we suggest that EROS may have detected up to 5low-amplification events due to halo lenses with mass $m\sim 10^{-7}~\sm$.Comment: uuencoded, gzip-ed postscript file (11 pages, including 8 figures). The paper has been thoroughly revised and updated throughout and assesses the most recent microlensing developments. The raw postscript file or LaTeX file and accompanying figures can be obtained via ftp to ftp://starsun1.ph.qmw.ac.uk/pub/ejk/ir+lens