Microlensing

This animation shows a binary star being lensed by a binary lens. The small black dots show the unperturbed positions of the red and blue stars, while the black circles show the size and location of the Einstein rings of the lenses taken individually. The sharp changes in the size of the lensed images of the stars lead to sharp changes in the brightness of the stars, which can be analyzed to determine many parameters about the objects doing the lensing, even though they are not bright enough to be seen.

Several experiments are know running to look for microlensing events toward the Large Magellanic Cloud and the bulge of the Milky Way. It appears that about one-half of the mass of the halo of the Milky Way is made of 0.5 solar mass MAssive Compact Halo Objects, or MACHOs.

Lensing events caused by binary lenses have been seen, and the sharp peaks predicted for caustic crossing are seen.

The graph shown above gives the predicted and observed brightness of a star in the Small Magellanic Cloud being magnified by a binary lens.

Notice how the star images are substantially displaced both long before and long after the dramatic brightening as the stars transit the Einstein rings. The behavior after the microlensing peak will be monitored by the Space Interferometry Mission (SIM), which will be launched in 2005.

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