Radio Telescope
Reveals Secrets Of Massive Black Hole
(23 April 2008) At the cores of many
galaxies, supermassive black holes expel powerful jets of particles at nearly
the speed of light.
Just how they perform this feat has long
been one of the mysteries of astrophysics. The leading theory says the
particles are accelerated by tightly-twisted magnetic fields close to the black
hole, but confirming that idea required an elusive close-up view of the jet's
inner throat. Now, using the unrivalled resolution of the National Radio
Astronomy Observatory's Very Long Baseline Array (VLBA), astronomers have
watched material winding a corkscrew outward path and behaving exactly as
predicted by the theory. "We have gotten the clearest look yet at the innermost
portion of the jet, where the particles actually are accelerated, and
everything we see supports the idea that twisted, coiled magnetic fields are
propelling the material outward," said Alan Marscher, of Boston University,
leader of an international research team. "This is a major advance in our
understanding of a remarkable process that occurs throughout the Universe," he
added.
Marscher's team studied a galaxy called BL Lacertae (BL Lac),
some 950 million light-years from Earth. BL Lac is a blazar, the most energetic
type of black-hole-powered galactic core. A black hole is a concentration of
mass so dense that not even light can escape its gravitational pull.
Supermassive black holes in galaxies' cores power jets of particles and intense
radiation in similar objects including quasars and Seyfert
galaxies.
Material pulled inward toward the black hole forms a
flattened, rotating disk, called an accretion disk. As the material moves from
the outer edge of the disk inward, magnetic field lines perpendicular to the
disk are twisted, forming a tightly-coiled bundle that, astronomers believe,
propels and confines the ejected particles. Closer to the black hole, space
itself, including the magnetic fields, is twisted by the strong gravitational
pull and rotation of the black hole.
Theorists predicted that material
moving outward in this close-in acceleration region would follow a
corkscrew-shaped path inside the bundle of twisted magnetic fields. They also
predicted that light and other radiation emitted by the moving material would
brighten when its rotating path was aimed most directly toward
Earth.
Marscher and his colleagues predicted there would also be a flare
later when the material hits a stationary shock wave called the "core" some
time after it has emerged from the acceleration region.
"That behaviour
is exactly what we saw," Marscher said, when his team followed an outburst from
BL Lac. In late 2005 and early 2006, the astronomers watched BL Lac with an
international collection of telescopes as a knot of material was ejected
outward through the jet. As the material sped out from the neighbourhood of the
black hole, the VLBA could pinpoint its location, while other telescopes
measured the properties of the radiation emitted from the knot.
Bright
bursts of light, X-rays, and gamma rays came when the knot was precisely at
locations where the theories said such bursts would be seen. In addition, the
alignment of the radio and light waves -- a property called polarisation --
rotated as the knot wound its corkscrew path inside the tight throat of twisted
magnetic fields.
"We got an unprecedented view of the inner portion of
one of these jets and gained information that's very important to understanding
how these tremendous particle accelerators work," Marscher said.
In
addition to the continent-wide VLBA, an array of 10 radio telescopes spread
from Hawaii to the Virgin Islands, the team used telescopes at the Steward
Observatory, the Crimean Astrophysical Observatory, Lowell Observatory, Perugia
University Astronomical Observatory, Abastumani Astrophysical Observatory,
NASA's Rossi X-Ray Timing Explorer, the University of Michigan Radio Astronomy
Observatory, and the Metsahovi Radio Observatory. The astronomers reported
their findings in the April 24 issue of the journal Nature.
The National
Radio Astronomy Observatory is a facility of the National Science Foundation,
operated under co-operative agreement by Associated Universities,
Inc.
(source: National Radio Astronomy Observatory)
