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Astron. Astrophys. 355, L39-L42 (2000)

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6. Discussion and conclusion

After 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 [FORMULA] objects. These were obtained from four different data sets analysed by four independent, cross-validated programs. So, the small number of observed events is unlikely to be due to bad detection efficiencies.

This allows us to put strong constraints on the fraction of the halo made of objects in the range [[FORMULA], [FORMULA]], excluding in particular at the 95% C.L. that more than 40% of the standard halo be made of objects with up to [FORMULA]. The preferred value quoted in Alcock et al. (1997a), [FORMULA] and [FORMULA], is incompatible with the limits in Fig. 2 at the 99.7% C.L. (but see the note added below).

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, [FORMULA], is 2.5 times lower than that of Alcock et al. (1997a) and is compatible with the limit presented here. None of the conclusions in this article have to be reconsidered. A detailed comparison of our results with those of Alcock et al. (2000) will be available in our forthcoming publication (Lasserre et al. 2000).

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000