"Great
Show-Off" Black Hole Is Producing Massive Shock Waves,
Cornell Astronomer Reports
Cornell University (http://www.cornell.edu)
Posted 1/18/2000
ATLANTA -- Something really shocking is going
on in a microquasar, or black hole, dubbed "Old
Faithful," some 40,000 light years from Earth. It seems
to be behaving like a giant particle collider, with massive
shock waves generating eruptions every 45 to 90 minutes.
This is the second time that Old Faithful,
the first known microquasar in our galaxy, the Milky Way,
has been observed to be acting strangely. Two years ago
astronomers presented evidence, from X-ray and infrared
observations, that the microquasar is sending out jets of
hot gas at close to regular half-hour intervals.
"The system is erratic enough to be
called chaotic," says Cornell University astronomer
Stephen Eikenberry.
Eikenberry presented the latest findings
from what he calls "the greatest show-off of all black
holes" in a poster-paper report at the national meeting
of the American Astronomical Society in Atlanta today (Jan.
14). Eikenberry's colleagues on the latest observations,
which were made in July 1998 with the Palomar Observatory
200-inch telescope and the Rossi X-ray Timing Explorer
satellite, were Keith Matthews of the California Institute
of Technology; Michael Muno, Ronald Remillard and Edward
Morgan of the Massachusetts Institute of Technology; and
Philip Blanco of the University of California, San Diego.
The discovery that a black hole is
producing explosive "shock fronts," says
Eikenberry, might be a clue to the behavior of much more
massive, extra-galactic black holes, called quasars, where
eruptions seem to occur on scales of years to decades. By
comparison, Old Faithful, a microquasar called GRS 1915+105
in the constellation Aquila (the eagle), has in the past
produced flares, or eruptions, as few as 20 seconds apart.
A black hole is a massive collapsed star
of such density that its intense gravitational pull prevents
even light from escaping. Like other collapsed stars, such
as neutron stars, black holes often are observed orbiting a
nearby star from which they are sucking in gaseous material.
This gas forms the accretion disk, a spiraling band of
material around the black hole. In some cases the black
hole appears to eject some of this
material in jets moving at nearly the speed of light.
The Old Faithful microquasar first
revealed itself as a bright X-ray source in 1992, and in
1994 radio observations found jets streaking away from the
black hole at more than 90 percent of the speed of light.
Two years later, infrared and X-ray observations revealed
smaller, rapid jet ejections occurring nearly every 30
minutes.
Now, says Eikenberry, a third class of
flare, made up of synchrotron radiation (the radiation
produced by fast-moving charged particles in a curved path),
is being emitted from an expanding plasma bubble. The
bubble, he says, is a cloud of high-energy electrons
swirling around in a magnetic field and possibly created by
shock waves. The magnetic field curves the trajectory of the
electrons and causes them to emit radiation. One or more
such plasma bubbles make up a jet ejection, theorizes
Eikenberry, an assistant professor of astronomy at Cornell.
What is new here, says Eikenberry, is a
different view of what can occur when material flows into a
rotating black hole. "Something happens at the
interface between the innermost part of the accretion disk
and the black hole that creates a jet. The innermost
accretion disk is somehow being affected by the black hole
and that causes part of the disk to be ejected in the form
of a jet and part of it to fall in," he says.
Previous observations showed a disruption
in the inner accretion disk followed by the appearance of a
jet ejection. These new observations, however, show the
plasma bubble appearing before the inner disk activity.
"Since we always thought that the
black hole in the inner disk drives jet activity, this was a
real surprise," says Eikenberry. "We have a few
possible ideas to explain it though. One idea is that a wave
of material, a shock front, is moving through the accretion
disk." Another theory, he says, is that the black hole
is emitting a continuous outflow of material and the
observed flares are shock fronts moving through the jets.
The newly discovered synchrotron radiation
jets "are clearly different from the jets previously
seen," says Eikenberry. "This is telling us there
are different ways to make jets or plasma bubbles in
microquasars."
Editor's Note: The original news release
can be found at http://www.news.cornell.edu/releases/Jan00/Eikenberry.blackholes.deb.html
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