Tracking the path of a black hole

Astronomers have for the first time measured the motion of a small black hole and a companion star speeding through our galactic neighborhood. The black hole and the star that it’s slowly devouring travel together on a looping path that ultimately will take them toward the outer reaches of our galaxy.

In the Sept. 13 Nature, researchers argue that the black hole, which is the compact remains of a massive star, was ejected from a star cluster. Ever since, it has been wandering the galaxy along with its stellar companion, the scientists propose.

Known as XTE J1118+480, the duo was discovered last year by NASA’s Rossi X-ray Timing Explorer satellite. Studies of its radio emissions reveal that the black hole-star combo qualifies as a miniature version of a quasar.

Material snared by the black hole from its companion has formed a swirling disk around the hole. Jets of subatomic particles spew from the disk, emitting radio waves. Quasars are believed to be powered by black holes millions to billions of times more massive than XTE J1118+480, and some emit jets of radio waves many time more intense.

Because of the pair’s proximity to Earth, astronomers were able to track the motion of the black hole and its partner with the Very Long Baseline Array, a network of radio telescopes that stretches from Hawaii to the U.S. Virgin Islands. The duo zips through space at 145 kilometers a second, notes study coauthor I. Felix Mirabel, who is affiliated with both the Institute for Astronomy and Space Physics in Buenos Aires and the French Atomic Energy Commission in Gif-sur-Yvette, France.

The orbit of XTE J1118+480 resembles that of globular clusters, large groupings of the oldest stars in the galaxy. Unlike most stars in the Milky Way, globular clusters don’t reside within the galaxy’s plane. Instead, they orbit the Milky Way’s center along paths that take them far below and above the plane.

The similar orbit of the black hole suggests that it long ago received a strong kick that expelled it from a globular cluster or from a region within the Milky Way’s disk. Simulations of that event suggest that a globular cluster is the most likely source of the black hole.

The massive star that collapsed to become the black hole “probably formed in a globular cluster even before our galaxy’s disk was formed,” Mirabel says. The observations therefore mark “the intense burst of star formation that took place during an early stage of our galaxy’s development,” he says.

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