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Astron. Astrophys. 355, L39-L42 (2000) 6. Discussion and conclusionAfter eight years of monitoring the Magellanic Clouds,
EROS has a meager crop of three microlensing candidates
towards the LMC and one towards the SMC
, whereas 27 events are expected for a spherical halo fully comprised
of This allows us to put strong constraints on the fraction of the
halo made of objects in the range [ What are possible reasons for such a difference? Apart from a potential bias in the detection efficiencies, several differences should be kept in mind while comparing the two experiments. First, EROS uses less crowded fields than MACHO with the result that blending is relatively unimportant for EROS . Second, EROS covers a larger solid angle (43 deg2 in the LMC and 10 deg2 in the SMC ) than MACHO, which monitors primarily the central 11 deg2 of the LMC . The EROS rate should thus be less contaminated by self-lensing that is more common in the central regions - the importance of self-lensing was first stressed by Wu (1994) and Sahu (1994). Third, the MACHO data have a more frequent time sampling. Finally, while the EROS limit uses both Clouds, the MACHO result is based only on the LMC . For halo lensing, the timescales towards the two Clouds should be nearly identical and the optical depths comparable. In this regard, we remark that the SMC event is longer than all LMC candidates from MACHO and EROS . Finally, given the scarcity of our candidates and the possibility that some observed microlenses actually lie in the Magellanic Clouds, EROS is not willing to quote at present a non zero lower limit on the fraction of the Galactic halo comprised of dark compact objects with masses up to a few solar masses. Note added. While the writing of this letter was being
finalised, the analysis of 5.7 yrs of LMC observations
by the MACHO group was made public (Alcock et al.
2000). The new favoured estimate of the halo mass fraction in the form
of compact objects, ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: March 21, 2000 ![]() |