Trent J. Perrotto
Headquarters, Washington
202-358-0321
trent.j.perrotto@nasa.gov
RELEASE: 12-009
NASA'S RXTE HELPS PINPOINT LAUNCH OF 'BULLETS' IN A BLACK HOLE'S JET
WASHINGTON -- Using observations from NASA's Rossi X-ray Timing
Explorer (RXTE) satellite and the National Science Foundation's (NSF)
Very Long Baseline Array (VLBA) radio telescope, an international
team of astronomers has identified the moment when a black hole in
our galaxy launched superfast knots of gas into space.
Racing outward at about one-quarter the speed of light, these
"bullets" of ionized gas are thought to arise from a region located
just outside the black hole's event horizon, the point beyond which
nothing can escape.
"Like a referee at a sports game, we essentially rewound the footage
on the bullets' progress, pinpointing when they were launched," said
Gregory Sivakoff of the University of Alberta in Canada. He presented
the findings today at the American Astronomical Society meeting in
Austin, Texas. "With the unique capabilities of RXTE and the VLBA, we
can associate their ejection with changes that likely signaled the
start of the process."
The research centered on the mid-2009 outburst of a binary system
known as H1743-322, located about 28,000 light-years away toward the
constellation Scorpius. Discovered by NASA's HEAO-1 satellite in
1977, the system is composed of a normal star and a black hole of
modest but unknown masses. Their orbit around each other is measured
in days, which puts them so close together that the black hole pulls
a continuous stream of matter from its stellar companion. The flowing
gas forms a flattened accretion disk millions of miles across,
several times wider than our sun, centered on the black hole. As
matter swirls inward, it is compressed and heated to tens of millions
of degrees, so hot that it emits X-rays.
Some of the infalling matter becomes re-directed out of the accretion
disk as dual, oppositely directed jets. Most of the time, the jets
consist of a steady flow of particles. Occasionally, though, they
morph into more powerful outflows that hurl massive gas blobs at
significant fractions of the speed of light.
In early June 2009, H1743-322 underwent this transition as astronomers
watched with RXTE, the VLBA, the Very Large Array near Socorro, N.M.,
and the Australia Telescope Compact Array (ATCA) near Narrabri in New
South Wales. The observatories captured changes in the system's X-ray
and radio emissions as the transformation occurred.
From May 28 to June 2, the system's X-ray and radio emissions were
fairly steady, although RXTE data show that cyclic X-ray variations,
known as quasi-periodic oscillations or QPOs, gradually increased in
frequency over the same period. On June 4, ATCA measurements showed
that the radio emission had faded significantly.
Astronomers interpret QPOs as signals produced by the interaction of
clumps of ionized gas in the accretion disk near the black hole. When
RXTE next looked at the system on June 5, the QPOs were gone.
The same day, the radio emission increased. An extremely detailed VLBA
image revealed a bright, radio-emitting bullet of gas moving outward
from the system in the direction of one of the jets. On June 6, a
second blob, moving away in the opposite direction, was seen.
Until now, astronomers had associated the onset of the radio outburst
with the bullet ejection event. However, based on the VLBA data, the
team calculated that the bullets were launched on June 3, about two
days before the main radio flare. A paper on the findings will be
published in the Monthly Notices of the Royal Astronomical Society.
"This research provides new clues about the conditions needed to
initiate a jet and can guide our thinking about how it happens," said
Chris Done, an astrophysicist at the University of Durham, England,
who was not involved in the study.
A super-sized version of the same phenomenon occurs at the center of
an active galaxy, where a black hole weighing millions to billions of
times our sun's mass can drive outflows extending millions of
light-years.
"Black hole jets in binary star systems act as fast-forwarded versions
of their galactic-scale cousins, giving us insights into how they
work and how their enormous energy output can influence the growth of
galaxies and clusters of galaxies," said lead researcher James
Miller-Jones at the International Center for Radio Astronomy Research
at Curtin University in Perth, Australia.
The Rossi X-ray Timing Explorer, which operated from Dec. 1995 to Jan.
2012, was managed by NASA's Goddard Space Flight Center in Greenbelt,
Md. The VLBA, the world's largest and highest-resolution astronomical
instrument, is controlled from the National Radio Astronomy
Observatory's Domenici Science Operations Center.
For images and animations related to this story, please visit:
http://www.nasa.gov/topics/universe/features/rxte-bullets.html
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