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Thursday, 13 September, 2001, 11:41 GMT 12:41 UK
Black hole's wild ride
The region of the black hole as seen from the Chandra observatory
By BBC News Online science editor Dr David Whitehouse

Astronomers have discovered an ancient black hole speeding through the Sun's galactic neighbourhood.

The rogue black hole is devouring a small companion star as the pair travel in an elliptical orbit that takes it from the outer reaches of our galaxy to its inner regions.

It is believed that the black hole is the remnant of a massive star that lived out its brief life billions of years ago and was then gravitationally kicked from its home star cluster to wander the Milky Way.

The discovery was made with observations from the US National Science Foundation's Very Long Baseline Array (Vlba) radio telescopes and the Rossi X-ray satellite in Earth orbit.

'Missing chapter'

The object, called XTE J1118+480, was discovered by the Rossi X-ray satellite on 29 March, 2000. Later observations with optical and radio telescopes showed that it is about 6,000 light-years from Earth.

Astronomers call it a microquasar, in which material sucked by the black hole from its companion star forms a hot, spinning disc that emits jets of subatomic particles that in turn give off radio waves.

The Rossi satellite
Observations pulled from various archives revealed data about the object spread over 43 years. The long timespan enabled astronomers to calculate the object's orbital path around the galactic centre.

"This discovery is the first step toward filling in a missing chapter in the history of our galaxy," said Felix Mirabel, an astrophysicist at the Institute for Astronomy and Space Physics of Argentina, and the French Atomic Energy Commission.

"We believe that hundreds of thousands of very massive stars formed early in the history of our galaxy, but this is the first black hole remnant of one of those huge primeval stars that we've found.

"This also is the first time that a black hole's motion through space has been measured," Mirabel added.

Stellar escape

A black hole is a concentration of mass with a pull of gravity so strong that not even light can escape.

"There are two possibilities [about how it may have got into its orbit]: either it formed in the galaxy's plane and was somehow kicked out of the plane, or it formed in a globular cluster and was kicked out of the cluster," said Vivek Dhawan, an astronomer at the National Radio Astronomy Observatory in Socorro, New Mexico.

Most of the stars in our Milky Way galaxy are within a thin disc, called the plane of the galaxy. However, globular clusters, each containing hundreds of thousands of the oldest stars in the galaxy, orbit the Milky Way's centre in paths that take them far from the plane. XTE J1118+480 orbits the galaxy's centre in a similar path.

Simulations of the gravitational interactions in globular clusters have shown that the black holes resulting from the collapse of the most massive stars should eventually be ejected from those clusters.

The research is reported in the journal Nature.

See also:

09 May 98 | Sci/Tech
Aussies spy 'birth' of black hole
13 Sep 00 | Sci/Tech
Middleweight black hole discovered
29 Mar 01 | Sci/Tech
Black hole excites astronomers
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