Microlensing towards the Large Magellanic Cloud: self-lensing for OGLE-II and OGLE-III

September 2011 • 2011MNRAS.416.1292C

Authors • Calchi Novati, S. • Mancini, L.

Abstract • We present an analysis of the results of the OGLE-III microlensing campaign towards the Large Magellanic Cloud (LMC). We evaluate for all the possible lens populations along the line of sight the expected microlensing quantities, number of events and 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 two OGLE-III microlensing candidates. In particular, we estimate the expected MW disc signal to be almost as large as that from LMC stars and are able, by itself, to explain the observed rate. We evaluate a 95 per cent confidence level (CL) upper limit for f, the halo mass fraction in the form of MACHOs, in the range 10-20 per cent for 10^-2 to 0.5 M_⊙, and f= 24 per cent for 1 M_⊙ (below 10 per cent in this full range and in particular below 5 per cent for 10^-2 to 0.1 M_⊙) for the Bright (All) samples of source stars. Furthermore, we find that these limits do not rise much even if we assume that the observed events are MACHOs. For the All sample, we also evaluate a rather significant constraint on f for larger values of the MACHO mass, in particular f∼ 50 per cent (95 per cent CL) for 100 M_⊙, 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 Expérience pour la Recherche d'Objets (EROS) collaborations, and we present a joint analysis of the OGLE-II and the OGLE-III campaigns.

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Sebastiano Calchi Novati

Associate Scientist