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

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5. Limits on Galactic halo Machos

EROS has observed four microlensing candidates towards the Magellanic Clouds, one from EROS 1 and two from EROS 2 towards the LMC , and one towards the SMC . As discussed in Palanque-Delabrouille et al. (1998), we consider that the long duration of the SMC candidate together with the absence of any detectable parallax, in our data as well as in that of the MACHO group (Alcock et al. 1997b), indicates that it is most likely due to a lens in the SMC . For that reason, the limit derived below uses the three LMC candidates; for completeness, we also give the limit corresponding to all four candidates.

The limits on the contribution of dark compact objects to the Galactic halo are obtained by comparing the number and durations of microlensing candidates with those expected from Galactic halo models. We use here the so-called "standard" halo model described in Palanque-Delabrouille et al. (1998) as model 1. The model predictions are computed for each EROS data set in turn, taking into account the corresponding detection efficiencies (Ansari et al. 1996; Renault et al. 1998; Afonso et al. 1999; Table 2 above), and the four predictions are then summed. In this model, all dark objects have the same mass M; we have computed the model predictions for many trial masses M in turn, in the range [[FORMULA], [FORMULA]].

The method used to compute the limit is as in Ansari et al. (1996). We consider two ranges of timescale [FORMULA], smaller or larger than [FORMULA] days. (All candidates have [FORMULA].) We can then compute, for each mass M and any halo fraction f, the combined Poisson probability for obtaining, in the four different EROS data sets taken as a whole, zero candidate with [FORMULA] and three or less (alt. four or less) with [FORMULA]. For any value of M, the limit [FORMULA] is the value of f for which this probability is 5%. Whereas the actual limit depends somewhat on the precise choice of [FORMULA], the difference ([FORMULA]) is noticeable only for masses around [FORMULA]. Furthermore, we consider 10 days to be a conservative choice.

Fig. 2 shows the 95% C.L. exclusion limit derived from this analysis on the halo mass fraction, f, for any given dark object mass, M. The solid line corresponds to the three LMC candidates; it is the main result of this letter. (The dashed line includes the SMC candidate in addition.) This limit rules out a standard spherical halo model fully comprised of objects with any mass function inside the range [FORMULA]. In the region of stellar mass objects, where this result improves most on previous ones, the new LMC data contribute about 60% to our total sensitivity (the SMC and EROS 1 LMC data contribute 15% and 25% respectively). The total sensitivity, that is proportional to the sum of [FORMULA] over the four EROS data sets, is 2.4 times larger than that of Alcock et al. (1997a). We observe that a large fraction of the domain previously allowed by Alcock et al. (1997a) is excluded by the limit in Fig. 2.

[FIGURE] Fig. 2. 95% C.L. exclusion diagram on the halo mass fraction in the form of compact objects of mass M, for the standard halo model ([FORMULA] inside 50 kpc), from all LMC and SMC EROS data 1990-98. The solid line is the limit inferred from the three LMC microlensing candidates; the dashed line includes in addition the SMC candidate. The MACHO 95% C.L. accepted region is the hatched area, with the preferred value indicated by the cross (Alcock et al. 1997a).

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000