Alternate Names

V821 Ara

Type

Stellar mass

Location

In the constellation Ara, the altar

Distance

27,000 light-years

Mass

5.8 to 10 times the mass of the Sun

Size

Diameter 20-35 miles (31-60 km), equal to the size of a large city

Discovery Methods

• Detection of an accretion disk
• Measuring the motions of a companion star

GX 339-4

GX 339-4 is one of the most active X-ray binaries yet discovered. Since 2000 it has produced four bright outbursts, each making the system shine dozens of times brighter than during its "quiet" phases. The outbursts suggest that the black hole periodically swallows larger blobs of material from its accretion disk.

The system consists of the black hole plus a companion star that is no more massive than the Sun. The two orbit each other once every 1.7 days, at a distance of about 10 million miles (16 million km). At such close range, the gravity of the black hole distorts the shape of its companion star, making it taper toward the black hole so that its profile resembles that of an egg.

The tapered end is so close to the black hole that hot gas flows off the star's surface and toward the black hole. The material forms a thin spiral known as an accretion disk.

Observations of GX 339-4 show that, at its closest, the densest material in the disk reaches to within about 20 miles (32 km) of the black hole's event horizon. At other times, though, it may retreat to about 600 miles (1,000 km) from the horizon. When that happens, a thinner but much hotter cloud of material appears to fill the inner region of the disk.

The difference is related to the flow of superhot gas, called a plasma, from the companion star. When the flow is strong, material spirals into the black hole quickly, so the thickest part of the disk moves closer to the event horizon. When the flow thins out, however, material spends more time in the inner disk so it's heated to billions of degrees, creating the thinner "mist" of material close to the black hole.

The disk also feeds jets of charged particles, which race into space from above the black hole's poles at close to the speed of light. The jets are powered by magnetic fields in the disk, although astronomers have yet to fully understand the process that generates the jets. They have discovered, however, that the jets in GX 399-4 continue to beam into space even when the flow from the companion star is low.

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This document was last modified: September 21, 2011.