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تسجيل مستخدم جديدMicrolensing towards the SMC: a new analysis of OGLE and EROS results
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S. Calchi Novati
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We present a new analysis of the results of the EROS-2, OGLE-II, and OGLE-III microlensing campaigns towards the Small Magellanic Cloud (SMC). Through a statistical analysis we address the issue of the emph{nature} of the reported microlensing candidate events, whether to be attributed to lenses belonging to known population (the SMC luminous components or the Milky Way disc, to which we broadly refer to as self lensing) or to the would be population of dark matter compact halo objects (MACHOs). To this purpose, we present profiles of the optical depth and, comparing to the observed quantities, we carry out analyses of the events position and duration. Finally, we evaluate and study the microlensing rate. Overall, we consider five reported microlensing events towards the SMC (one by EROS and four by OGLE). The analysis shows that in terms of number of events the expected self lensing signal may indeed explain the observed rate. However, the characteristics of the events, spatial distribution and duration (and for one event, the projected velocity) rather suggest a non-self lensing origin for a few of them. In particular we evaluate, through a likelihood analysis, the resulting upper limit for the halo mass fraction in form of MACHOs given the expected self-lensing and MACHO lensing signal. At 95% CL, the tighter upper limit, about 10%, is found for MACHO mass of $10^{-2} mathrm{M}_odot$, upper limit that reduces to above 20% for $0.5 mathrm{M}_odot$ MACHOs.
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duration. In particular we consider lensing by massive compact halo objects (MACHOs) in the dark matter haloes of both the Milky Way (MW) and the LMC, and self lensing by stars in the LMC bar and disc, in the MW disc and in the stellar haloes of both the LMC and the MW. As a result we find that the self-lensing signal is able to explain the 2 OGLE-III microlensing candidates. In particular, we estimate the expected MW disc signal to be almost as large as that from LMC stars and able, by itself, to explain the observed rate. We evaluate a 95% CL emph{upper} limit for $f$, the halo mass fraction in form of MACHOs, in the range 10-20% for $(10^{-2}-0.5) mathrm{M}_odot$, and $f=24%$ for $1 mathrm{M}_odot$ (below 10% in this full range, and in particular below 5% for $(10^{-2}-0.1) mathrm{M}_odot$) for the Bright (All) samples of source stars. Furthermore, we find that these limits do not rise much even if we assume the observed events emph{are} MACHOs. For the All sample we also evaluate a rather significant constraint on $f$ for larger values of the MACHO mass, in particular $fsim 50%$ (95% CL) for $100 mathrm{M}_odot$, to date the stronger bound coming from microlensing analyses in this mass range. Finally, we discuss these results in the framework of the previous observational campaigns towards the LMC, that of the MACHO and the EROS collaborations, and we present a joint analysis of the OGLE-II and the OGLE-III campaigns.
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We present the results from the OGLE-II survey (1996-2000) towards the Large Magellanic Cloud (LMC), which has the aim of detecting the microlensing phenomena caused by dark matter compact objects in the Galactic Halo (Machos). We use high resoluti
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