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Title: The MACHO Project Large Magellanic Cloud microlensing results from the first two years and the nature of the galactic dark halo

Abstract

The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected ({approximately}1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2{lt}{cflx t}{lt}200 days to be {tau}{sub 2}{sup 200}=2.9{sub {minus}0.9}{sup +1.4}{times}10{sup {minus}7} based upon our eight event sample. This exceeds the optical depth, {tau}{sub backgnd}=0.5{times}10{sup {minus}7}, expected from known stars, and the difference is to be compared with the optical depth predicted for a {open_quotes}standard{close_quotes} halo composed entirely of MACHOs: {tau}{sub halo}=4.7{times}10{sup {minus}7}. To compare with Galactic halo models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo {open_quotes}background{close_quotes}). This gives a fairly model-independent estimatemore » of the halo mass in MACHOs within 50 kpc of 2.0{sub {minus}0.7}{sup +1.2}{times}10{sup 11}M{sub {circle_dot}}, which is about half of the {open_quotes}standard halo{close_quotes} value. We also find a most probable MACHO mass of 0.5{sub {minus}0.2}{sup +0.3}M{sub {circle_dot}}, although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10{sup {minus}4}M{sub {circle_dot}} to 0.03M{sub {circle_dot}} contribute {approx_lt}20{percent} of the {open_quotes}standard{close_quotes} dark halo. {copyright} {ital 1997} {ital The American Astronomical Society}« less

Authors:
 [1];  [2];  [3];  [4];  [5];  [6];  [1];  [4]; ; ;  [7];  [8]
  1. Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)|[Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States)
  2. Supercomputing Facility, Australian National University, Canberra, ACT 0200 (Australia)
  3. Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)|[Department of Physics, University of California, Davis, California 95616 (United States)
  4. Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611 (Australia)
  5. Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States)|[Departments of Astronomy and Physics, University of Washington, Seattle, Washington 98195 (United States)
  6. Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)|[Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States)|[Department of Physics, University of California, Davis, California 95616 (United States)|[Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556 (United States)
  7. Center for Particle Astrophysics, University of California, Berkeley, California 94720 (United States)|[Department of Physics, University of California, San Diego, La Jolla, California 92093 (United States)
  8. Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)|[Center for Particle Astrophysics, University of California (United States)
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
OSTI Identifier:
628715
DOE Contract Number:  
W-7405-ENG-48
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 486; Journal Issue: 2; Other Information: PBD: Sep 1997
Country of Publication:
United States
Language:
English
Subject:
66 PHYSICS; MILKY WAY; GRAVITATIONAL LENSES; MAGELLANIC CLOUDS; NONLUMINOUS MATTER; PHOTOMETRY; SPATIAL DISTRIBUTION; MASS DISTRIBUTION

Citation Formats

Alcock, C., Allsman, R.A., Alves, D., Axelrod, T.S., Becker, A.C., Bennett, D.P., Cook, K.H., Freeman, K.C., Griest, K., Guern, J., Lehner, M.J., and Marshall, S.L.. The MACHO Project Large Magellanic Cloud microlensing results from the first two years and the nature of the galactic dark halo. United States: N. p., 1997. Web. doi:10.1086/304535.
Alcock, C., Allsman, R.A., Alves, D., Axelrod, T.S., Becker, A.C., Bennett, D.P., Cook, K.H., Freeman, K.C., Griest, K., Guern, J., Lehner, M.J., & Marshall, S.L.. The MACHO Project Large Magellanic Cloud microlensing results from the first two years and the nature of the galactic dark halo. United States. doi:10.1086/304535.
Alcock, C., Allsman, R.A., Alves, D., Axelrod, T.S., Becker, A.C., Bennett, D.P., Cook, K.H., Freeman, K.C., Griest, K., Guern, J., Lehner, M.J., and Marshall, S.L.. Mon . "The MACHO Project Large Magellanic Cloud microlensing results from the first two years and the nature of the galactic dark halo". United States. doi:10.1086/304535.
@article{osti_628715,
title = {The MACHO Project Large Magellanic Cloud microlensing results from the first two years and the nature of the galactic dark halo},
author = {Alcock, C. and Allsman, R.A. and Alves, D. and Axelrod, T.S. and Becker, A.C. and Bennett, D.P. and Cook, K.H. and Freeman, K.C. and Griest, K. and Guern, J. and Lehner, M.J. and Marshall, S.L.},
abstractNote = {The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected ({approximately}1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2{lt}{cflx t}{lt}200 days to be {tau}{sub 2}{sup 200}=2.9{sub {minus}0.9}{sup +1.4}{times}10{sup {minus}7} based upon our eight event sample. This exceeds the optical depth, {tau}{sub backgnd}=0.5{times}10{sup {minus}7}, expected from known stars, and the difference is to be compared with the optical depth predicted for a {open_quotes}standard{close_quotes} halo composed entirely of MACHOs: {tau}{sub halo}=4.7{times}10{sup {minus}7}. To compare with Galactic halo models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo {open_quotes}background{close_quotes}). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of 2.0{sub {minus}0.7}{sup +1.2}{times}10{sup 11}M{sub {circle_dot}}, which is about half of the {open_quotes}standard halo{close_quotes} value. We also find a most probable MACHO mass of 0.5{sub {minus}0.2}{sup +0.3}M{sub {circle_dot}}, although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10{sup {minus}4}M{sub {circle_dot}} to 0.03M{sub {circle_dot}} contribute {approx_lt}20{percent} of the {open_quotes}standard{close_quotes} dark halo. {copyright} {ital 1997} {ital The American Astronomical Society}},
doi = {10.1086/304535},
journal = {Astrophysical Journal},
number = 2,
volume = 486,
place = {United States},
year = {1997},
month = {9}
}