A Caustic View of Halo Microlensing

Abstract

The only microlensing events towards the Magellanic Clouds for which the location of the lens is strongly constrained are the two binary caustic crossing events. In at least one and possibly both cases, the lens lies at, or close to, the clouds themselves. On the face of it, this seems an improbable occurrence if the Galactic dark halo provides the bulk of the lensing population, as suggested by standard analyses of the MACHO dataset towards the Large Magellanic Cloud (LMC). We use a binomial statistic to assess the prior probability of observing $M$ non-halo binary caustic events given a total sample of $N$ caustic binaries. We generalize for the case of multi-component Galactic and Magellanic Cloud models the Bayesian likelihood method for determining the lens mass and halo fraction from the observed timescales. We introduce a new statistic, the ``outcome discriminator'', which measures the consistency between the binary caustic data, prior expectation, and the MACHO LMC dataset as a whole. If the clouds are not embedded in their own dark halos of MACHOs, then the discovery of two non-halo caustic binary events out of two ($M=N=2$) is inconsistent with expectation given the MACHO dataset. Galactic models in which $M=1$ is the likeliest outcome are also inconsistent with the data, though models in which $M=1$ has a reasonable prior probability are not. We consider the possibilities that the Magellanic Clouds are embedded in dark haloes of their own, or that the Galactic halo is intrinsically deficient in the binary systems which produce caustic crossing events. Either of these possibilities provide greater compatibility between observation and prior expectation, though the idea of Magellanic haloes is perhaps the more natural of the two and has support from kinematical studies.

To appear in ApJ. 31 pages, 13 figures

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• Binary Microlensing Events from the MACHO Project
• Self-Lensing Models of the LMC
• Gravitational lensing in fourth order gravity
• Theory of pixel lensing towards M31 I: the density contribution and mass of MACHOs
• The Origin of the Microlensing Events Observed Towards the LMC and the Stellar Counterpart of the Magellanic Stream
• Lightcurve Classification in Massive Variability Surveys
• The So-Called "Bar" in the Large Magellanic Cloud
• Lightcurve Classification in Massive Variability Surveys II: Transients towards the Large Magellanic Cloud
• MACHO 96-LMC-2: Lensing of a Binary Source in the LMC and Constraints on the Lensing Object
• Parallax Microlensing Events in the OGLE II Database Toward the Galactic Bulge
• Is the Large Magellanic Cloud a Large Microlensing Cloud?
• On the Nature and Location of the Microlenses
• Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter
• Magnitude bias of microlensed sources towards the Large Magellanic Cloud
• Probing the mass function of halo dark matter via microlensing
• On the Intrinsic Bias in detecting Caustic Crossings between Galactic Halo and Self-lensing Events in the Magellanic Clouds