404 Gravitational lensing
Figure 14.6.Microlensing event observed by the MACHO collaboration in their first year
of data towards the LMC. The event lasted about 33 days. The data are shown for blue
light, red light and the ratio red light to blue light, which for perfect achromaticity should
be equal to one (from [38]).
In September 1993, the French collaboration EROS (Exp ́erience de Recher-
che d’Objets Sombres) [37] announced the discovery of two microlensing
candidates, and the American–Australian collaboration MACHO (for the
collaboration they use the same acronym as for the compact objects) of one
candidate [38] by monitoring several millions of stars in the LMC (figure 14.6).
The MACHO team went on to report the observation of 13 to 17 events (one
being a binary lensing event; see figure 4.5) by analysing their 5.7 year of LMC
data [39]. The inferred optical depth isτ= 1. 2 +−^00 ..^43 × 10 −^7 with an additional
20% to 30% of systematic error. Correspondingly, this implies that about 20% of
the halo dark matter is in the form of MACHOs with a most likely mass in the
range 0.15–0.9M depending on the halo model. Moreover, it might well be that
not all the MACHOs are in the halo: some could be stars in the LMC itself or