A Binary Lensing Event Toward the LMC: Observations and Dark Matter Implications
Publication information:
D. P. Bennett, C. Alcock, R. A. Allsman, D. Alves, T. S. Axelrod, A. Becker, K. H. Cook, K. C. Freeman, K. Griest, J. Guern, M. J. Lehner, S. L. Marshall, D. Minniti, B. A. Peterson, M. R. Pratt, P. J. Quinn, S. H. Rhie, A. W. Rodgers, C. W. Stubbs, W. Sutherland, and D. Welch. 1996. “A Binary Lensing Event Toward the LMC: Observations and Dark Matter Implications”. Nuclear Physics B Proceedings Supplements, Vol. 51, 51, Pp. 152. doi:10.1016/S0920-5632(96)00497-5
Abstract
The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of τmeas = 2.9+1.4-0.9 × 10-7. This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: τback = 5.4 × 10-8, but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass ~ 0.4Msolar. One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved, and the measured caustic crossing time allows us to estimate the distance to the lenses. Under the assumption that the source star is a single star and not a short period binary, we show that the lensing objects are very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.