Image: Black hole illustration
M. Weiss / CXC / NASA
This is an artist's representation of GRO J1655-40, a binary star system observed in April 2005 by Chandra. This binary consists of a black hole and a normal star, shown in blue. Gas is being pulled away from the star to form a red disk spinning around the black hole. The black hole's magnetic field causes the disk to lose some of its angular momentum, allowing material to fall in.
updated 6/21/2006 2:03:25 PM ET 2006-06-21T18:03:25

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 Thursday's 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.

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.

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 "It would not be a major surprise if all accretion disks rely on internal magnetic properties, at least partially."

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