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Cosmic speeding. Artist's impression of the black hole GRO J1655-40, which is orbited by a normal companion star.

Black Hole Caught Speeding

Astronomers have clocked a black hole racing through the flattened disk of the Milky Way galaxy at 400,000 kilometers per hour. This stupendous velocity provides additional support for the popular belief that black holes are the remains of detonated stars and might shed light on what sends them flying through space.

Black holes are thought to form when the dense core of a supernova--a massive, exploding star--collapses in on itself. If this collapse is slightly skewed, or if the exploding star is orbiting a companion star, the remaining stellar corpse might be flung violently through space. Last year, astronomers found a speedy black hole (ScienceNOW, 13 September 2001), but that hole resides in the Milky Way's extended halo, where high velocities are the rule--making the case for a violent origin inconclusive. In contrast, the new black hole moves four times as fast as stars in its part of the galaxy, suggesting that the high speed must result from a supernova kick.

Félix Mirabel of the French Atomic Energy Commission in Saclay and his colleagues tracked black hole GRO J1655-40 across the sky by determining the position of its dim companion star in 1996 and again in 2001 using the Hubble Space Telescope. Combining this tiny displacement with the hole's previously determined velocity along the line of sight as it zooms toward Earth yielded its velocity in three dimensions. In today's issue of Astronomy and Astrophysics, the team reports that GRO J1655-40 is screaming through space at 400,000 kilometers per hour--100 times faster than a rifle bullet. "I can't explain such a high velocity in any other way than through a supernova explosion," says Mirabel.

Ultimately the team hopes to trace GRO J1655-40's path back to its origin, which would reveal its age and shed light on the properties of the progenitor star. They speculate that the black hole might have formed in a stellar cluster in the constellation Scorpius. But nailing that down will be difficult because the distance of the black hole is poorly known, which makes mapping its trajectory difficult.

"This is a beautiful piece of work," says Gijs Nelemans of the University of Cambridge, U.K. He's particularly interested in the potential insight into the nature of the kick that sends black holes flying through space.

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Background information on black holes