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Friday, 14 January, 2000, 11:58 GMT
Lone drifter black holes discovered

A Black Hole may have passed in front of this star A Black Hole may have passed in front of this star

By BBC News Online Science Editor Dr David Whitehouse

Some black holes drift alone through the Galaxy rather than waltzing around companion stars, astronomers have discovered.

The research was done by two international teams of astronomers using Nasa's Hubble Space Telescope and ground-based telescopes in Australia and Chile.

All previously known star-sized black holes have been found in orbit around normal stars, with their presence betrayed by their effect on the companion star. The two new black holes were detected indirectly, by the way their gravity bends the light of a more distant star behind them.

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

Powerful lens

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 too small to be detected with current instruments.

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

Careful analysis of the two events revealed that the black holes are each about six times heavier than the Sun.

If the objects were ordinary stars they would be bright enough to outshine the more distant background source star. The masses are also too large to be the well-known white dwarfs or neutron stars. This leaves black holes as the best explanation.

Missing matter

This microlensing detection technique, combined with the Hubble Space Telescope's ability 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'. This the 90% of the Universe's material which must exist but cannot be seen.

The microlensing events were discovered in 1996 and 1998 by the Massive Compact Halo Object (Macho) collaboration.

The Macho team surveys tens of millions of stars in the direction of the centre of our galaxy. The starfield is very crowded here and this increases the chances for seeing the 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.

Follow-up observations were made with Hubble on 15 June, 1999, to clearly identify the lensed star for the first event and make a precise measurement of its brightness after the lensing event.

The Hubble image indicates that the lensed star was blended with two neighbouring stars of similar brightness that 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.

So far there have been more than 300 cases of gravitational microlensing seen towards the central regions of our galaxy to date. Astronomers say they represent a rich harvest for future discoveries.

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See also:
28 Oct 99 |  Sci/Tech
Hubble homes in on black hole
17 Aug 99 |  Sci/Tech
Black hole detected swallowing matter
16 Nov 99 |  Sci/Tech
The home-made black hole

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