Radio telescope reveals secrets of massive black hole
Apr 23, 2008 - 4:00:00 AM
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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 polarization -- rotated
as the knot wound its corkscrew path inside the tight throat of
twisted magnetic fields.
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By National Radio Astronomy Observatory,
[RxPG]
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 unrivaled resolution
of the National Radio Astronomy Observatory's Very Long Baseline
(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 behavior 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 neighborhood 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 polarization -- 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.
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