Binary microlensing event OGLE-2009-BLG-020 gives verifiable mass, distance, and orbit predictions

J. Skowron, A. Udalski, A. Gould, Subo Dong, L. A G Monard, C. Han, C. R. Nelson, J. McCormick, D. Moorhouse, G. Thornley, A. Maury, D. M. Bramich, J. Greenhill, S. Kozłowski, I. Bond, R. Poleski, L. Wyrzykowski, K. Ulaczyk, M. Kubiak, M. K. SzymańskiG. Pietrzyński, I. Soszyński, B. S. Gaudi, J. C. Yee, L. W. Hung, R. W. Pogge, D. L. Depoy, C. U. Lee, B. G. Park, W. Allen, F. Mallia, J. Drummond, G. Bolt, A. Allan, P. Browne, N. Clay, M. Dominik, S. Fraser, K. Horne, N. Kains, C. Mottram, C. Snodgrass, I. Steele, R. A. Street, Y. Tsapras, F. Abe, D. P. Bennett, C. S. Botzler, D. Douchin, M. Freeman, A. Fukui, K. Furusawa, F. Hayashi, J. B. Hearnshaw, S. Hosaka, Y. Itow, K. Kamiya, P. M. Kilmartin, A. Korpela, W. Lin, C. H. Ling, S. Makita, K. Masuda, Y. Matsubara, Y. Muraki, T. Nagayama, N. Miyake, K. Nishimoto, K. Ohnishi, Y. C. Perrott, N. Rattenbury, To Saito, L. Skuljan, D. J. Sullivan, T. Sumi, D. Suzuki, W. L. Sweatman, P. J. Tristram, K. Wada, P. C M Yock, J. P. Beaulieu, P. Fouqué, M. D. Albrow, V. Batista, S. Brillant, J. A R Caldwell, A. Cassan, A. Cole, K. H. Cook, Ch Coutures, S. Dieters, D. Dominis Prester, J. Donatowicz, S. R. Kane, D. Kubas, J. B. Marquette, R. Martin, J. Menzies, K. C. Sahu, J. Wambsganss, A. Williams, M. Zub

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80 Citations (Scopus)


We present the first example of binary microlensing for which the parameter measurements can be verified (or contradicted) by future Doppler observations. This test is made possible by a confluence of two relatively unusual circumstances. First, the binary lens is bright enough (I = 15.6) to permit Doppler measurements. Second, we measure not only the usual seven binary-lens parameters, but also the "microlens parallax" (which yields the binary mass) and two components of the instantaneous orbital velocity. Thus, we measure, effectively, six "Kepler+1" parameters (two instantaneous positions, two instantaneous velocities, the binary total mass, and the mass ratio). Since Doppler observations of the brighter binary component determine five Kepler parameters (period, velocity amplitude, eccentricity, phase, and position of periapsis), while the same spectroscopy yields the mass of the primary, the combined Doppler + microlensing observations would be overconstrained by 6 + (5 + 1) - (7 + 1) = 4 degrees of freedom. This makes possible an extremely strong test of the microlensing solution. We also introduce a uniform microlensing notation for single and binary lenses, define conventions, summarize all known microlensing degeneracies, and extend a set of parameters to describe full Keplerian motion of the binary lenses.

Original languageEnglish
Article number87
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 1 Sep 2011
Externally publishedYes



  • binaries: general
  • Galaxy: bulge
  • gravitational lensing: micro

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Skowron, J., Udalski, A., Gould, A., Dong, S., Monard, L. A. G., Han, C., Nelson, C. R., McCormick, J., Moorhouse, D., Thornley, G., Maury, A., Bramich, D. M., Greenhill, J., Kozłowski, S., Bond, I., Poleski, R., Wyrzykowski, L., Ulaczyk, K., Kubiak, M., ... Zub, M. (2011). Binary microlensing event OGLE-2009-BLG-020 gives verifiable mass, distance, and orbit predictions. Astrophysical Journal, 738(1), [87].