Journal of Astronomy and Space Sciences

The Korean Space Science Society (한국우주과학회)
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Are the Distribution of Einstein Crossing Times of Galactic Microlensing Events Bimodal?

  • Struble, Mitchell F. (Depatment of Physics and Astronomy, University of Pennsylvania) ;
  • Wickramasinghe, Thulsi (Fulbright Fellow at the University of Peradeniya)
  • Received : 2012.03.02
  • Accepted : 2012.05.10
  • Published : 2012.06.15
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Abstract

The observed distribution of a blending-corrected sample of Einstein ring crossing times, $t_E$, for microlensing events toward the galactic bulge/bar are analyzed. An inspection of the distribution of crossing times suggests that it may be bimodal, indicating that two populations of lenses could be responsible for observed microlensing events. Given the possibility that microlensing in this direction can be due to the two most common classes of stars, main-sequence and white dwarf, we analyze and show via Monte Carlo simulations that the observed bimodality of $t_E$ can be derived from their accepted mass functions, and the density distributions of both stellar populations in the galactic disk and bulge/bar, with a transverse velocity distribution that is consistent with the density distribution. Kolmogorov-Smirnov (KS) one sample tests shows that a white dwarf population of about 25% of all stars in the galaxy agrees well with the observed bimodality with a KS significance level greater than 97%. This is an expanded and updated version of a previous investigation (Wickramasinghe, Neusima, & Struble, in Mao 2008). A power-point version of the talk, with introductory figures, is found at: https://sites.google.com/site/rhkochconference/agenda-1/program.

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References

  1. Alcock C, Allsman RA, Alves D, Axelrod TS, Bennett DP, et al., First observation of parallax in a gravitational microlensing event, ApJ, 454, L125-L128 (1995). http://dx.doi.org/10.1086/309783
  2. Alcock C, Allsman RA, Alves DR, Axelrod TS, Becker AC, et al., The MACHO project: microlensing optical depth toward the galactic bulge from difference image analysis, ApJ, 541, 734-766 (2000). http://dx.doi.org/10.1086/309484
  3. Alcock C, Allsman RA, Alves DR, Axelrod TS, Becker AC, et al., Direct detection of a microlens in the Milky Way, Natur, 414, 617-619 (2001). http://dx.doi.org/10.1038/414617a
  4. Bennett DP, Becker AC, Calitz JJ, Johnson BR, Laws C, et al., The microlensing event MACHO-99-BLG-22/OGLE-1999-BUL-32: an intermediate mass black hole, or a lens in the bulge, ApJ, 579, 639-659 (2002). http://arxiv.org/abs/astro-ph/0207006 https://doi.org/10.1086/342225
  5. Binney J, Merrifield M, Galactic astronomy (Princeton University Press, Princeton, 1998), Chapter 3.
  6. Bochanski JJ, Hawley SL, Covey KR, West AA, Reid IN, et al., The luminosity and mass functions of the low-mass stars in the galactic disk. II. The field, AJ, 139, 2679-2699 (2010). http://dx.doi.org/10.1088/0004-6256/139/6/2679
  7. Chabrier G, Galactic stellar and substellar initial mass function, PASP, 115, 763-795 (2003). http://dx.doi.org/10.1086/376392
  8. Einstein A, Lens-like action of a star by the deviation of light in the gravitational field, Sci, 84, 506-507 (1936). http://dx.doi.org/10.1126/science.84.2188.506
  9. Han C, Gould A, The mass spectrum of MACHOs from parallax measurements, ApJ, 447, 53-61 (1995). http://dx.doi.org/10.1086/175856
  10. Han C, Gould A, Statistical determination of the MACHO mass spectrum, ApJ, 467, 540-557 (1996). http://dx.doi.org/10.1086/177631
  11. Kallivayalil N, Patten BM, Marengo M, Alcock C, Werner MW, et al., Identification of the microlens in event MACHO-LMC-20, ApJ, 652, L97-L100 (2006). http://dx.doi.org/10.1086/510293
  12. Kepler SO, Kleinman SJ, Nitta A, Koester D, Castanheira BG, et al., White dwarf mass distribution in the SDSS, MNRAS, 375, 1315-1324 (2007). http://dx.doi.org/10.1111/j.1365-2966.2006.11388.x
  13. Kroupa P, Tout CA, Gilmore G, The distribution of low-mass stars in the Galactic disc, MNRAS, 262, 545-587 (1993). https://doi.org/10.1093/mnras/262.3.545
  14. Mao S, Introduction to microlensing, in Proceedings of the Manchester Microlensing Conference: The 12th International Conference and ANGLES Microlensing Workshop, Manchester, UK, 21-25 Jan 2008, PoS(GMC8)002 (2008).
  15. Mao S, Smith MC, Wozniak P, Udalski A, Szymanski M, et al., Optical gravitational lensing experiment OGLE-1999-BUL-32: the longest ever microlensing event - evidence for a stellar mass black hole?, MNRAS, 329, 349-354 (2002). http://dx.doi.org/10.1046/j.1365-8711.2002.04986.x
  16. Paczynski B, Gravitational microlensing by the galactic halo, ApJ, 304, 1-5 (1986). http://dx.doi.org/10.1086/164140.
  17. Scalo JM, The stellar initial mass function, FCPh, 11, 1-178 (1986).
  18. Sumi T, Abe F, Bond IA, Dodd RJ, Hearnshaw JB, et al., Microlensing optical depth toward the galactic bulge from microlensing observations in astrophysics group observations during 2000 with difference image analysis, ApJ, 591, 204-227 (2003). http://dx.doi.org/10.1086/375212
  19. Sumi T, Kamiya K, Udalski A, Bennett DP, Bond IA, et al., Unbound or distant planetary mass population detected by gravitational microlensing, Natur, 473, 349-352 (2011). http://dx.doi.org/10.1038/nature10092
  20. Sumi T, Wozniak PR, Udalski A, Szymanski M, Kubiak M, et al., Microlensing optical depth toward the Galactic bulge using bright sources from OGLE-II, ApJ, 636, 240-260 (2006). http://dx.doi.org/10.1086/497951