Magnetic Fields Nudge Matter Into Black Holes

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.]


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."