Black holes
are known for their strong gravitational tugs, but gravity alone isn't enough
to send matter tumbling into the center of one.
Magnetism
provides the final nudge, a new study finds.
The
research, detailed in the July 22 issue of the journal Nature, confirms
a theory first put forth in 1973 that magnetic fields drive both the infall of matter into black holes and the production of
light energy created by the process.
Getting
rid of angular momentum
A black
hole's gravity is enough to draw matter in and keeps it spinning in a stable
"accretion" disk. But before it can take that final plunge, the
material must lose some of its rotation speed, called angular momentum.
"Many
people are familiar with the phrase 'bodies at rest tend to stay at rest, and
bodies motion tend to stay in motion,'" said study team member Jon Miller,
an astronomer at the University of Michigan. "The same thing is true for
orbiting bodies--they tend to stay in stable orbits, unless acted upon by a
force."
If angular
momentum from the disk were not dissipated away, gas in the accretion disk
would circle the black hole forever in a stable orbit, like the planets around
our sun.
Magnetism's
role
Using
NASA's Chandra
X-ray Observatory, the researchers studied GRO
J1655-40, a binary system made up of a seven-solar-mass black hole that is
stealing gas from the surface of a normal star. The siphoned gas accumulates in
an accretion disk around the black hole.
The
spinning gas generates its own magnetic field, and this field powers a "wind"
of charged particles blowing away from the black hole.
The wind,
which Chandra detected, transfers angular momentum from the inner regions of
the disk outward. This slows down some of the spinning gas, allowing it to fall
onto the black hole. [Click here
for an animation.]
The
magnetic field also causes turbulence and friction to build up within the disk.
The friction heats up the gas to millions of degrees, causing it to glow
brilliantly in the ultraviolet and X-ray
bands.
Anything
with a disk
The
researchers believe magnetic fields play an important role in the activities of
black holes of all sizes, whether they are stellar-mass ones whose accretion
disks are fed by companion stars, or even galaxy-anchoring supermassive
monsters whose disks are formed from the stellar winds of multiple stars.
The finding
should also apply to other objects that have accretion disks, such as neutron
stars and white
dwarfs, Miller said.
"We
already know that disks around some young stars are driven by [magnetic]
processes," Miller told SPACE.com. "It would not be a major
surprise if all accretion disks rely on internal magnetic properties, at least
partially."
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