The MACHO Project Large Magellanic Cloud Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo
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abstract
The MACHO Project is a search for dark matter in the form of massive compact
halo objects (Machos). Photometric monitoring of millions of stars in the Large
Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is
used to search for gravitational microlensing events caused by these otherwise
invisible objects. Analysis of the first 2.1 years of photometry of 8.5 million
stars in the LMC reveals 8 candidate microlensing events. This is substantially
more than the number expected ($\sim 1.1$) from lensing by known stellar
populations. The timescales ($\that$) of the events range from 34 to 145 days.
We estimate the total microlensing optical depth towards the LMC from events
with $2 < \that < 200$ days to be $\tau_2^{200} = 2.9 ^{+1.4}_{-0.9} \ten{-7}$
based upon our 8 event sample. This exceeds the optical depth, $\tau_{\rm
backgnd} = 0.5 \ten{-7}$, expected from known stars, and the difference is to
be compared with the optical depth predicted for a ``standard" halo composed
entirely of Machos: $\tau_{halo} = 4.7\ten{-7}$. Likelihood analysis gives a
fairly model independent estimate of the halo mass in Machos within 50 kpc of
$2.0^{+1.2}_{-0.7} \ten{11} \msun$, about half of the ``standard halo" value.
We also find a most probable Macho mass of $0.5^{+0.3}_{-0.2}\msun$, although
this value is strongly model dependent. Additionally, the absence of short
duration events places stringent upper limits on the contribution of low-mass
Machos: objects from $10^{-4} \msun$ to $0.03 \msun$ contribute $\simlt 20\%$
of the ``standard" dark halo.
