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Black Holes Encyclopedia

V4641 Sagittarii

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Alternate Names

SAX J1819.3-2525


Stellar mass


in the constellation Sagittarius

Finder Chart


24,000 to 40,000 light-years (7,400-12,300 parsecs)


3 to 10 times the mass of the Sun


Diameter roughly 11-37 miles (18-60 km)

Discovery Methods


V4641 Sagittarii

V4641 Sagittarii (Sgr) that originally was thought to be the nearest black hole to Earth, with a distance of just 1,600 light-years. Continued observations of the system, though, have pushed it out to several times that distance — a minimum of 24,000 light-years.

Astronomers learned of the black hole’s presence in 1999, when the system flared up. X-ray and radio telescopes detected powerful “jets” of charged particles racing away from the system at almost the speed of light.

V4641 Sgr consists of a black hole and a “normal” companion star. The black hole is stealing hot gas from the companion. When enough gas builds up in a disk around the black hole, it produces a large outburst that can cause the system to shine much brighter than normal. Much of the extra energy is in the form of X-rays, so V4641 is classified as an X-ray nova.

On the night of September 15, 1999, Australian astronomer Ron Stubbings noticed that the system was about two magnitudes (six times) brighter than normal. He notified other astronomers by email. One team of astronomers quickly aimed the Rossi X-Ray Timing Explorer (RXTE) satellite at the star, and discovered that it was producing hundreds of times more X-ray energy than normal, suggesting that a powerful event had taken place. Over the next two days, RXTE detected three more bright flare-ups. For a while, the system was the brightest X-ray object in the entire sky.

Other astronomers studied the system with radio telescopes around the world. As they watched over a series of days, they saw “jets” of electrically charged particles streaming away from the system. The jets and the bright X-ray flare-ups, which grew hundreds of times brighter and fainter in a matter of seconds or minutes, suggested that the companion to the normal star was a black hole. Other types of dense stars, like white dwarfs and neutron stars, cannot produce the same type of phenomena.

Astronomers later calculated that the jets were squirting into space from the inner portion of the accretion disk around the black hole, quite near its event horizon. Particles in the jets were accelerated to about 90 percent of lightspeed by powerful magnetic fields, and blasted out into space just before they would have fallen into the black hole.

Because of the jet and bright accretion disk, V4641 Sgr is classified as a “microquasar.” Full-size quasars are disks of hot gas surrounding supermassive black holes in the cores of distant galaxies. Microquasars are much smaller, but they are powered by the same mechanism and they produce large amounts of high-energy gamma rays and X-rays.



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This document was last modified: February 13, 2012.


Artist's impression of stellar-mass black hole
Artist's Rendering

Artist's impression of an X-ray nova
Artist's Rendering


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