Dark loop. The black hole emits energy by consuming a faint companion star (purple) and wanders above and below the galaxy's plane, where our sun (yellow) resides.

Black Hole's Galactic Loop-the-Loop

Astronomers have for the first time charted the path of a black hole, by measuring energy from a star perpetually circling it. The black hole's trajectory suggests that the tightly bound pair was ejected from an ancient cluster of stars billions of years ago, according to a report in the 13 September issue of Nature.

The binary system, called x-ray nova XTE J1118+480, was first spotted by the Rossi X-ray Timing Explorer last year. The satellite detected x-rays from hot gas that passes from a faint companion star into the lightweight black hole. The system, 6000 light-years away, flared brightly enough for astronomers to pinpoint its position in May and July 2000 with the Very Long Baseline Array, a network of 10 radio dishes across the U.S. and its territories. The researchers also found the companion star on older sky-survey photos taken 43 years apart.

These positions allowed astronomer Vivek Dhawan of the National Radio Astronomy Observatory in Socorro, New Mexico, and his colleagues to trace out the black hole's orbit. It is radically different from those of the sun and most other stars in the Milky Way. Rather than circling within the flat plane of the galaxy's disk, the pair dives above and below the disk. This giant loopy path resembles the motions of globular clusters, ancient swarms of stars that wander in random patterns around the Milky Way. The team's data, published in Nature, suggest that the system was born in a globular cluster and then spat out via gravitational encounters with other black holes, propelling it onto a new orbit through the galaxy.

Astrophysicist Shrinivas Kulkarni of the California Institute of Technology in Pasadena admires the calculation of the orbit, but he doesn't think it was kick-started inside a globular cluster. His research suggests that when black holes fling each other out of globular clusters, they probably don't pair up with tiny companion stars. Instead, Kulkarni suspects that the black hole and its partner were blasted out of the galaxy's disk by an unbalanced supernova explosion.

Whatever got it started, XTE J1118+480 probably isn't alone: At least 10,000 ejected black holes may wander through the galaxy on similar paths, says Dhawan, although most are farther away and may not have companion stars to reveal their presence.

Related sites

News release with orbital animations
Very Long Baseline Array
Technical paper on detection of black hole

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