Twinkle, twinkle, microlens: in trying to probe the dark matter surrounding, the Milky Way, astronomers have confirmed the identity of a nearby gravitational lens

The next step was to find the mass and distance of the intervening dark object. On May 13, 1999, Alcock and his team made a color-composite image of the location of the LMC-5 event with the Hubble Space Telescope, and discovered a faint, red star almost right on top of a bright, blue star. Their angular separation was less than one-seventh of an arc-second--the equivalent of viewing a penny from almost twenty miles away. The faint, red star, the team concluded, was the likely lensing object, because it would not have moved far in the intervening six years, though, of course, it was no longer in the line of sight of the blue background star.

That conclusion, however, led to another problem. The luminosity and spectral characteristics of the red star showed that it is about 11 percent the mass of the Sun, at a distance of about 2,000 light-years from Earth. The mathematical models of the microlensing event, however, told an entirely different story: the estimated lens mass would be only 4 percent the mass of the Sun--smaller than any known star--and it would lie only 600 light-years from Earth. The nature of the LMC-5 lens remained indeterminate.

Now, theory and observation have finally converged. Andrew Gould, an astronomer at Ohio State University in Columbus, reworked the gravitational-lens equations and showed that a second, previously unrecognized solution to the equations exists, which places the lensing object at a distance of 2,000 light-years rather than 600. A few weeks after Gould's result was published, Andrew Drake at Princeton University, Kem Cook of Lawrence Livermore National Laboratory in California, and Stefan C. Keller at the Australian National University in Canberra published new measurements of the LMC-5 object, again made with the Hubble telescope. The new measurements confirmed that the red star was moving in a way consistent with Gould's calculations.

What those findings mean is that the MACHO that caused the LMC-5 lensing event is indeed this unnamed red star. It's the first definitively confirmed identity of a MACHO lens.

Ironically, the first confirmed "massive compact halo object" isn't a galactic halo object at all. At a distance of 2,000 light-years, in the direction of the Large Magellanic Cloud, this faint little red star actually lies in the outskirts of the Milky Way disk--one among billions of other nondescript dwarf stars, traveling in an unremarkable orbit within the galaxy. As so often happens in life, it owes its fame to chance: for 75.6 days in 1993, it was in the right place at the right time.

CHARLES LIU is a professor of astrophysics at the City University of New York and an associate at the American Museum of Natural History.

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