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For Release: June 11, 1997
Nasa Scientist Details Surprising Findings On Celestial Jets That Turn On And Off Like A Faucet In The Skies
Powerful beams or "jets" of radiation in our Milky Way Galaxy that turn on and off like a faucet are among intriguing phenomena discovered by scientists and detailed today at a national astronomy conference.
These jets of material are spewing from two black holes at velocities near the speed of light. The two jet-producing, nearby black holes represent one of the hottest new classes of celestial objects discovered in recent years.
Since the early part of the 20th century, astronomers and physicists have trained their telescopes to see jets in the far reaches of space. But just three years ago, researchers surprisingly found the jets lurking in the Milky Way. Undetected at one moment, then appearing the next, the jets eject vast amounts of material into nearby space.
Dr. Alan Harmon, an astrophysicist at NASA's Marshall Space Flight Center in Huntsville, Ala., presented the findings of the NASA team to the American Astronomical Society meeting at the Benton Convention Center in Winston-Salem, N.C.
For many years, astronomers have observed jets being fired at extremely high velocity away from the centers of galaxies millions of light years away. Scientists knew that deep in the nucleus of these galaxies, something works to accelerate particles to very high energy.
"The only way we know to supply that energy," said Harmon, "is from the immense gravitational well of a black hole" -- a massive and extremely compact object, created from the collapse of a star or collection of stars. A black hole has such a powerful gravitational force that not even light can escape from it.
But because of surrounding stars and dust, and their great distance from Earth, it is exceedingly difficult for astronomers to see into the center of a galaxy to learn the details of what goes on.
Astronomers needed a way to observe these mysterious jets at close range.
That opportunity came with the powerful telescope orbiting aboard NASA's Compton Gamma Ray Observatory, which is known as the Burst and Transient Source Experiment. That instrument, and the space-based Russian observatory Granat, just three years ago allowed astronomers to discover two X-ray sources "in our own galaxy -- practically in the neighborhood -- that were accelerating matter in the form of jets," said Harmon.
"The jets can be seen in radio telescopes as a string of bright 'blobs' that appear to move across the sky in a few hours. These sources do not represent the powerful central engine in the nucleus of a galaxy, but are a special breed of binary X-ray source which we knew little about until recently," the scientist said.
Harmon and the NASA team believe these jets emanate from the vicinity of a black hole with a mass of seven to 30-times the size of the Sun. The jets are thought to reside in a binary system, along with a low-mass star, from which the black hole accretes, or collects, matter. The matter forms an accretion disk surrounding the black hole.
"An unusual thing about the jets is that they do not seem to be common to all black hole binaries," said Harmon, "or we probably would have known about them earlier.
"The other unusual thing, of course, is that whenever the X-ray source flares into a bright state, the jets become visible in the radio band. And, at other times, they are not seen at all. This transient production is puzzling, and is still not understood by theorists," Harmon said.
So scientists wonder: If a black hole binary can make jets occasionally, what are the conditions necessary for jets to be produced? "The answer," Harmon believes, "may be in the accretion disk and surrounding regions. The trigger for jet production clearly seems to be connected to the appearance of X-rays and gamma rays. That tells us the material has been, or is flowing from the star into the accretion disk. Accreting material may be feeding the jets."
The NASA researchers feel fortunate that the two X-ray sources discovered in the Milky Way remain active. The sources are monitored daily from the ground and in space by a host of observatories.
"Observers from around the world are also looking for new objects with jets," said Harmon. "And in the very near future, we may clear the way for understanding these unusual binary systems -- and possibly even the central engines of active galaxies."
NOTE TO EDITORS: Additional information on celestial jets can be obtained at the website: