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Donald Savage
Headquarters, Washington, DC January 7, 1998
(Phone: 202/358-1547) EMBARGOED UNTIL 9:30 AM EST
William Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-5017)
RELEASE: 98-2
"OLD FAITHFUL" BLACK HOLE EJECTS MASS EQUAL TO AN ASTEROID
Scientists observing a disk of matter surrounding a black
hole in our galaxy have discovered that the disk is periodically
disrupted and hurled outward in opposite directions from the black
hole, in jets moving at nearly the speed of light. The black hole
replenishes the disk by pulling hot gas from the surface of a
nearby "companion" star, and then undergoes another disruption,
repeating the sequence at half-hour intervals.
The researchers represent teams at the California Institute
of Technology, Pasadena, CA, the Massachusetts Institute of
Technology (MIT), Boston, and NASA's Goddard Space Flight Center,
Greenbelt, MD, which all worked to correlate the disappearance of
X-ray emitting hot gas in the disk with the appearance, shortly
thereafter, of rapidly expanding jets. Dr. Ronald Remillard of
MIT and Dr. Jean Swank of Goddard are presenting X-ray results,
obtained with NASA's Rossi X-ray Timing Explorer (RXTE), which
show the disappearing disk. Dr. Stephen Eikenberry of Caltech is
presenting new infrared observations which demonstrate that when
the X-rays from the disk vanish, the jets suddenly appear. The
observations will be the subject of a press conference by three of
the researchers involved, to be held Jan. 7 during the winter
meeting of the American Astronomical Society in Washington, DC.
The disks of hot gas, known as accretion disks, are
commonly observed around black holes with orbiting stellar
companions, but the near simultaneous disappearance of the disk
and formation of the jet has never been seen before. It promises
to shed light on the origin of the enigmatic jets, also commonly
observed near accreting black holes, but poorly understood.
"The system behaves like the celestial version of Old
Faithful," notes Dr. Craig Markwardt, a researcher working with
Swank at Goddard. "At fairly regular intervals, the accretion
disk is disrupted and a fast moving jet is produced."
"This jet is staggeringly more powerful than a geyser,"
adds Swank. "Every half-hour, the black hole, in the
constellation Aquila, throws off the mass equal to that of a 100
trillion ton asteroid at nearly the speed of light (approximately
650 million miles per hour). This process clearly requires a lot
of energy -- each cycle is equivalent to six trillion times the
annual energy consumption of the entire United States."
"What is even more amazing is that we are seeing the first
clues to the source of matter ejected in the jets -- the
correlations we discovered indicate that the jet material must
come from the inner disk. For years theorists have hypothesized
that the jets come from somewhere close to the black hole, but no
one had ever actually seen that direct link until now" said
Eikenberry.
Black holes are very massive objects with gravitational
fields so intense that near them, nothing, not even light, can
escape their pull. While this prevents anyone from observing
black holes directly, their presence can be inferred from effects
on nearby matter. Many of the known or suspected black holes are
orbiting a close "companion" star. The black hole's gravity pulls
gas from the companion star into a swirling disk of material which
orbits around the black hole, much as soap suds swirl around a
bathtub drain. As it falls into the black hole, the gas in the
disk is compressed and heated to millions of degrees, emitting
X-rays.
"We found that the X-rays from the disk disappeared almost
completely every half hour. About five minutes later, they would
return again. The results of the X-ray observations, Eikenberry's
infrared observations at Mt. Palomar, CA, and infrared and radio
observations made by Dr. Felix Mirabel, Director, Centre d'Etudes
de Saclay, France, and collaborators, showed that very shortly
after the X-rays vanished, jets appeared at infrared and radio
wavelengths. Since X-ray emitting hot gas in the disk disappears
during these episodes, we all concluded that the matter in the
disk must have escaped, probably flung out in the jets. As the
black hole pulls more gas from its companion star, the disk
appears again, along with the X-rays, and the cycle starts over,"
Swank said. Dr. Mirabel obtained results with the United Kingdom
Infrared Telescope and the Very Large Array in New Mexico.
"The great value of these observations," notes Swank, RXTE
project scientist, "is that potentially they are giving us a key
to answer some big questions. It is remarkable how little we
understand of these jets, because they are such a common and
impressive phenomenon. In fact, black hole jet formation is
rather mysterious because it's hard to understand how anything can
be expelled from the vicinity of a black hole since its powerful
gravity pulls everything in. I believe this black hole will prove
to be one key to finding out lots of things about black holes and jets.
"This is like having a miniature quasar in your back yard;
and, because it is much smaller, it changes over minutes and
hours, rather than months and years. This will let us learn a lot
in a much shorter period of time," said Elkenberry.
- end -
Note to Editors: A video file, with computer animation and, will
be broadcast on NASA TV Jan. 7. Images to support this story are
available on the Internet at:
FTP://PAO.GSFC.NASA.GOV/newsmedia/JAN_AAS/BH
RXTE information is available at:
http://heasarc.gsfc.nasa.gov/docs/xte/XTE.html