A wide-ranging array of radio dishes trained on the supermassive black hole at the center of our galaxy has revealed a glimpse of the magnetic field close to the Milky Way’s dark heart. The results could help explain why black holes, although not emitting any light themselves, are able to make the churning gas and dust around them shine with the brightness of thousands of stars.
The gravity of a black hole is so intense that, once something has passed inside, it can never escape; not even light can get out. This makes black holes themselves almost impossible to observe. But the strong gravity also makes the area immediately around a black hole into a bright and extremely lively place.
Typically black holes are surrounded by an accretion disk, a flat swirl of orbiting material. When any of that material gets too close to the edge of the hole, known as the event horizon, its very atoms are ripped apart. Whereas the nuclei disappear below the horizon, the much lighter electrons get caught up in the black hole’s intense magnetic field. They whiz around at high speed, spiraling around the magnetic field lines. This twisting motion causes them to emit photons, known as synchrotron radiation, which is the main source of emission from matter close to the black hole.
“Magnetism is implicated in all the energy extracted from black holes: synchrotron radiation, jets. We’re just beginning to tease that out,” says Shep Doeleman, assistant director of the Massachusetts Institute of Technology’s Haystack Observatory in Westford, Massachusetts, and principal investigator of the Event Horizon Telescope (EHT).
The black hole at the center of our Milky Way galaxy, dubbed Sagittarius A* (Sgr A*) is a relatively quiescent beast, but nonetheless the material around it shines extremely brightly. Other black holes, variously called quasars or active galactic nuclei, shine brightly enough to be seen across the universe, and they produce hugely powerful jets of material that fly out of their poles. To understand how these cosmic monsters produce such jets requires knowledge of black holes’ magnetic fields. But up until now it hasn’t been possible to map them close in to the event horizon.
Now, thanks to recent advances in data processing and electronics—including the graphics processors used in video games—the EHT has given us a glimpse of what Sgr A*’s magnetic field looks like. The EHT is an effort to harness together existing radio telescopes scattered across the globe and use them as one giant instrument. The farther apart the dishes, the finer the detail in the final images. A separation as wide as Earth allows will be necessary to see the relatively tiny disk of Sgr A*. Despite weighing as much as 4 million suns, the black hole at the center of our galaxy is only 12 million kilometers across, less than the distance from the sun to Mercury. Viewing it from Earth is like studying something smaller than a golf ball on the moon. To increase its resolution, the EHT will eventually include dishes as far apart as Hawaii and France and Greenland and the South Pole. “We’re creating a new kind of instrument, which couldn’t have been imagined a decade ago,” Doeleman says.
For the current study, researchers used a prototype EHT, which included observatories in Hawaii, California, and Arizona. The array detects radio waves with a wavelength of 1.3 millimeters, which can penetrate clouds of dust and gas that shroud the galactic center. This is shorter than what most traditional radio telescopes can detect, but it is dead on the wavelength at which Sgr A*’s disk shines brightest.
In the new study, reported online today in Science, the team was not looking for the brightness of the radio waves but rather their polarization. The electrons twirling around magnetic field lines emit polarized photons, revealing the structure of the magnetic field. Although they were not able to produce a map of the field, the researchers could see some areas where it was a tangle of disordered lines and other areas where the lines were ordered, all within about six times the radius of the event horizon.
The disordered areas could be showing turbulence in the accretion disk, says team leader Michael Johnson of the Harvard-Smithsonian Center for Astrophysics. This would suggest that most of the brightness of Sgr A* is coming from this very small region close to the event horizon. The more ordered polarization, he says, may indicate where the field is stretched out—possibly the source of jets.
How black holes and other objects form jets is a very contentious topic in astronomy. Results such as this “will help us beat down the complexity of astrophysical models,” says Heino Falcke of Radboud University Nijmegen in the Netherlands, who is involved with the EHT but not this study. “For the very first time we will see the footprint of jet formation. Looking right down to the event horizon is the key to understanding.”
Astrophysicist Roger Blandford of Stanford University in Palo Alto, California, agrees. “Seeing a strong magnetic field is very encouraging. It strengthens the case that jets are powered electromagnetically,” he says. “The idea that you can actually look at these things … it’s an incredible thing to do.”