Jan 14, 2000: Two international teams of astronomers using NASA's Hubble Space Telescope and ground-based telescopes in Australia and Chile have discovered the first examples of isolated stellar-mass black holes adrift among the stars in our galaxy.

All previously known stellar black holes have been found in orbit around normal stars, with their presence determined by their effect on the companion star. The two isolated black holes were detected indirectly by the way their extreme gravity bends the light from a more distant star behind them.

Right: A & B Two images of a crowded starfield as seen through a ground-based telescope show the subtle brightening of a star due to the effect of gravitational microlensing, where an invisible but massive foreground object passes in front of the star and amplifies its light. The dark lensing object is estimated to be a six-solar-mass black hole that is drifting alone among the stars. Right: C  A NASA Hubble Space Telescope image of the same field clearly resolves the lensed star and yields its true brightness. [more information]

"These results suggest that black holes are common, and that many massive but normal stars may end their lives as black holes instead of as neutron stars," said David Bennett of the University of Notre Dame, South Bend, IN. Bennett presented his team's results yesterday in Atlanta at the 195th meeting of the American Astronomical Society.

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The findings also suggest that stellar mass black holes do not require some sort of interaction in a double star system to form, but may also be produced in the collapse of isolated massive stars, as has long been proposed by stellar theorists.

The black hole's gravity acts like a powerful lens, bending the light of a background star so that it appears as two separate images when the black hole slowly drifts in front of it. The bending angle is about 100 times smaller than the angular resolution of Hubble, so the two distorted images of the background star cannot be separated even in high-resolution Hubble images.

However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. Bennett's team was searching for these passages, also called gravitational microlensing events.

Careful analysis of the two events reveals that the lensing objects are each approximately six times the mass of the Sun. If the objects were ordinary stars with this mass they would be bright enough to outshine the more distant background source star. The masses are also too large to be white dwarfs or neutron stars. This leaves a black hole as the best explanation.

Above: A diagram showing gravitational microlensing by a black hole. Light shines from a distant star and passes near enough to a black hole on the way to Earth. The black hole bends the light, which produces microlensed images. These images appear as a single brightened star to an observer on Earth.

This microlensing detection technique, combined with Hubble's extraordinary resolution to pinpoint the lensed star, opens the possibility for searching for lone black holes and assessing whether they contribute to the galaxy's long-sought "dark matter."

These microlensing events were discovered in 1996 and 1998 by the Massive Compact Halo Object (MACHO) collaboration with the National Science Foundation, using the 1.3 meter telescope at the Mt. Stromlo Observatory in Canberra, Australia, while the magnification was still increasing. The prompt discovery and announcement of these events enabled precise follow-up observations by the Global Microlensing Alert Network from the .9 meter telescope at Cerro Tololo Inter-American Observatory and by the Microlensing Planet Search project using the 1.9 meter telescope at Mt. Stromlo.

The MACHO team surveys tens of millions of stars in the direction of the center of our galaxy, where the star field is very crowded, increasing the chances for seeing rare gravitational microlensing events. The two events were also of exceptionally long duration, lasting 800 and 500 days respectively, which suggests that the lensing objects have a high mass.

Right: This diagram shows various positions of Earth with respect to a distant star and a somewhat closer black hole. Only when Earth, the black hole, and the star are aligned will an observer on Earth see a brightened image of the distant star.

Follow-up observations were done with Hubble to clearly identify the lensed star for the first event and make a precise measurement of its brightness after the lensing event. The Hubble frame indicates that the lensed star was blended with two neighboring stars of similar brightness which could not be separated in the poorer-resolution, ground-based images. Hubble's identification of the lensed star allowed for an accurate estimate of the mass of the black hole.

The 1998 event was brighter, and modeling of the ground-based measurements enabled astronomers to determine the brightness of the lensed star, but this determination awaits confirmation with future Hubble images.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

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