GS 1354-64

GS 1354-64 appears to be the farthest black hole yet discovered in the Milky Way galaxy, at a distance of at least 85,000 light-years. If so, then it is also the brightest "quiet" black hole yet seen.

The system, which consists of a black hole plus a "normal" star in a tight orbit around each other, was discovered by Ginga, an X-ray satellite, during an outburst in 1987. It took about two decades to develop reliable estimates of its distance and mass, however, in part because of its location in the sky. It appears near the plane of the Milky Way galaxy as seen from Earth, so we view the system through thick clouds of gas and dust. That absorbs and reddens the system's light, making it more difficult to see and study.

Several sets of observations with an 8.2-meter telescope in Chile allowed astronomers to obtain the first reliable distance measurements. The observations also revealed that the companion star is probably about 1-2 times the mass of the Sun. It appears to be nearing the end of its life, however, so it has puffed up to a few times the Sun's diameter.

As the star grew larger, its surface moved closer to the black hole. The star's gravitational grip on its surface layers also weakened, allowing the black hole to pull gas away from it. As the gas funnels toward the black hole it forms a wide, thin disk, known as an accretion disk.

The flow of material into the accretion disk and on into the black hole isn't smooth, like the flow of a water hose, however. Instead, it appears to be clumpy, like half-stirred pancake batter, so the brightness of the accretion disk varies considerably. At times, the black hole appears to ingest a large clump of material at once, producing a powerful outburst of X-rays. The system was discovered during one of these outbursts, and a smaller one followed a decade later.

The accretion disk's flickering has made it impossible for astronomers to measure the angle at which we view the system, so they cannot determine a precise mass for the black hole. They can, however, calculate a minimum mass of about eight times the mass of the Sun, with a few additional solar masses possible.

GS 1354 appears to reside about 4,000 light-years above the Milky Way's flat disk, in an extended bubble of space known as the halo. The chemistry of the companion star, however, suggests that the system was born inside the disk. It may have moved into the halo when the star that gave birth to the black hole exploded as a supernova, propelling the binary system across the galaxy like a high-powered rocket.

Astronomers are trying to solve several vexing questions about GS 1354. For one thing, even when the accretion disk is not producing outbursts of X-rays, it is much brighter than any other known disk around a star-sized black hole. Such mysteries are difficult to solve for such a remote system.

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This document was last modified: May 11, 2012.