MACHO 96-LMC-2: Lensing of a Binary Source in the Large Magellanic Cloud and Constraints on the Lensing Object

We present photometry and analysis of the microlensing alert MACHO 96-LMC-2 (event LMC-14 in an earlier paper). This event was initially detected by the MACHO Alert System and subsequently monitored by the Global Microlensing Alert Network (GMAN). The ~3% photometry provided by the GMAN follow-up effort reveals a periodic modulation in the light curve. We attribute this to binarity of the lensed source. Microlensing fits to a rotating binary source magnified by a single lens converge on two minima, separated by Δχ² ~ 1. The most significant fit X1 predicts a primary which contributes ~100% of the light, a dark secondary, and an orbital period (T) of ~9.2 days. The second fit X2 yields a binary source with two stars of roughly equal mass and luminosity and T = 21.2 days. Observations made with the Hubble Space Telescope (HST) resolve stellar neighbors which contribute to the MACHO object's baseline brightness. The actual lensed object appears to lie on the upper LMC main sequence. We estimate the mass of the primary component of the binary system, M ~ 2 M_☉. This helps to determine the physical size of the orbiting system and allows a measurement of the lens proper motion. For the preferred model X1, we explore the range of dark companions by assuming 0.1 M_☉ and 1.4 M_☉ objects in models X1a and X1b, respectively. We find lens velocities projected to the LMC in these models of _X1a = 18.3 ± 3.1 km s^-1 and _X1b = 188 ± 32 km s^-1. In both these cases, a likelihood analysis suggests an LMC lens is preferred over a Galactic halo lens, although only marginally so in model X1b. We also find _X2 = 39.6 ± 6.1 km s^-1, where the likelihood for the lens location is strongly dominated by the LMC disk. In all cases, the lens mass is consistent with that of an M dwarf. Additional spectra of the lensed source system are necessary to further constrain and/or refine the derived properties of the lensing object. The LMC self-lensing rate contributed by 96-LMC-2 is consistent with model self-lensing rates. Thus, even if the lens is in the LMC disk, it does not rule out the possibility of Galactic halo microlenses altogether. Finally, we emphasize the unique capability of follow-up spectroscopic observations of known microlensed LMC stars, combined with the nondetection of binary source effects, to locate lenses in the Galactic halo.