Observations of the Binary Microlens Event MACHO 98-SMC-1 by the Microlensing Planet Search Collaboration

We present observations of the binary lensing event MACHO 98-SMC-1 conducted at the Mount Stromlo 1.9 m telescope by the Microlensing Planet Search (MPS) collaboration. The MPS data constrain the first caustic crossing to have occurred after 1998 June 5.55 UT and thus directly rule out one of the two fits presented by the PLANET collaboration (model II). This substantially reduces the uncertainty in the relative proper-motion estimations of the lens object. We perform joint binary microlensing fits of the MPS data together with the publicly available data from the EROS, MACHO/GMAN, and OGLE collaborations. We also study the binary lens fit parameters previously published by the PLANET and MACHO/GMAN collaborations by using them as initial values for χ²-minimization. Fits based on the PLANET model I appear to be in conflict with the GMAN-CTIO data. From our best fit, we find that the lens system has a proper motion of μ = 1.5 ± 0.3 km s^-1 kpc^-1 with respect to the source, which implies that the lens system is most likely to be located in the Small Magellanic Cloud, strengthening the conclusion of previous reports.