The OGLE View of Microlensing towards the Magellanic Clouds. II. OGLE-II
  SMC data
  The primary goal of this paper is to provide the evidence that can either
prove or falsify the hypothesis that dark matter in the Galactic halo can clump
into stellar-mass compact objects. If such objects existed, they would act as
lenses to external sources in the Magellanic Clouds, giving rise to an
observable effect of microlensing. We present the results of our search for
such events, based on the data from the second phase of the OGLE survey
(1996-2000) towards the SMC. The data set we used is comprised of 2.1 million
monitored sources distributed over an area of 2.4 square degrees. We found only
one microlensing event candidate, however its poor quality light curve limited
our discussion on the exact distance to the lensing object.
  Given a single event, taking the blending (crowding of stars) into account
for the detection efficiency simulations, and deriving the HST-corrected number
of monitored stars, the microlensing optical depth is tau=(1.55+-1.55)10e-7.
This result is consistent with the expected SMC self-lensing signal, with no
need of introducing dark matter microlenses. Rejecting the unconvincing event
leads to the upper limit on the fraction of dark matter in the form of MACHOs
to f<20 per cent for deflectors' masses around 0.4 Msun and f<11 per cent for
masses between 0.003 and 0.2 Msun (95 per cent confidence limit). Our result
indicates that the Milky Way's dark matter is unlikely to be clumpy and form
compact objects in the sub-solar-mass range.
