Magnetism helps black holes blow off gas

Black holes are a bit like babies when they eat: Some food goes in, and some gets flung back out into space. Astronomers now say they understand how these meals become so messy — and it’s a trait all black holes share, no matter their size.

Magnetic fields drive the turbulent winds that blow gas away from black holes, says Keigo Fukumura, an astrophysicist at James Madison University in Harrisonburg, Va. Using X-rays emitted from a relatively small black hole siphoning gas from a nearby star, Fukumura and colleagues traced the winds flowing from the disk of stellar debris swirling around the black hole. Modeling these winds showed that magnetism, not other means, got the gas moving in just the right way.
The model was previously used to explain the way winds flow around black holes millions of times the mass of the sun. Showing that the model now also works for a smaller stellar-mass black hole suggests that magnetism may drive winds in black holes of all sizes. These results, published online March 6 in Nature Astronomy, could give clues to how black holes consume and expel matter and also to why some galaxies stop forming stars.

Astronomers first proposed that magnetic fields powered the winds around black holes in the 1970s, but the idea has been controversial. Directly observing the winds is impossible. Their existence is inferred by a black hole’s X-ray spectrum — an inventory of light broken up by wavelength.

In 2005, astronomers used the Chandra X-ray Observatory to capture the X-ray spectrum of a relatively puny black hole with seven times the mass of the sun. The companion star it feeds on has about twice the heft of the sun. The system, called GRO J1655-40, is about 11,000 light-years away in the constellation Scorpius.

GRO J1655-40’s X-rays revealed its turbulent winds. Some astronomers argued the data provided evidence that powerful magnetic fields fueled the winds. Others, however, suggested the winds resulted from extremely hot gas swirling around the black hole.

“I think the new paper clears this controversy up,” says Andrew Fabian, an astrophysicist at the University of Cambridge who was not involved with the new study. The model Fukumura developed, he says, is extremely detailed and accounts for characteristics of GRO J1655-40’s X-ray spectrum that other models can’t explain.
Features of the spectrum, for example, suggest that the winds are dense and move moderately quickly, but don’t blow far from the black hole. That matches models of magnetically fueled wind. Models related to the heat of the gas alone make the winds blow too far.

The magnetic fields form from the electric current generated by electrons and protons swirling in a pancake-shaped accretion disk. Parts of the disk spin around the black hole at different speeds, which amplifies the fields. That, in turn, turns the accretion disk into a vortex, pulling matter into the black hole and fueling winds that blow some of it outward.

“A good fraction of the mass actually gets kicked out of the black hole,” Fukumura says. “If it didn’t get thrown off, we wouldn’t see it.”

The magnetic fields probably arc around the black hole from pole to pole. But no one knows for sure because they are hard to detect. Recently, the Event Horizon Telescope, which pointed several telescopes at the center of the Milky Way, did spot patterns in the way the light of our galaxy’s central, supermassive black hole was oriented that signaled it has magnetic fields. Astronomers plan to use the telescope array to search for more evidence of magnetic fields around black holes next month.

Studying the magnetic fields of black holes reveals information about the structure of their accreting disks and the winds that blow from them. “Winds from black hole disks can be very powerful,” Fabian notes. “In the case where the black hole is massive and at the center of a galaxy, the wind can push all the gas out of the host galaxy, stopping further star formation and causing the galaxy to appear red and dead.”