The MACHO project 2nd year LMC microlensing results and dark matter implications

Abstract

The MACHO Project is searching for galactic dark matter in the form of massive compact halo objects (Machos). Millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge are photometrically monitored in an attempt to detect rare gravitational microlensing events caused by otherwise invisible Machos. Analysis of two years of photometry on 8.5 million stars in the LMC reveals 8 candidate microlensing events, far more than the $\sim1$ event expected from lensing by low-mass stars in known galactic populations. From these eight events we estimate the optical depth towards the LMC from events with $2 < \that < 200$ days to be $\tau_2^{200} \approx 2.9 ^{+1.4}_{-0.9} \ten{-7}$. This exceeds the optical depth of $0.5\ten{-7}$ expected from known stars and is to be compared with an optical depth of $4.7\ten{-7}$ predicted for a ``standard'' halo composed entirely of Machos. The total mass in this lensing population is $\approx 2^{+1.2}_{-0.7} \ten{11} \msun$ (within 50 kpc from the Galactic center). Event timescales yield a most probable Macho mass of $0.5^{+0.3}_{-0.2}\msun$, although this value is quite model dependent.