A black hole’s gravity can heat up the disk around it to shine brightly, but what powers the jets some of them produce remains a mystery.

A black hole’s gravity can heat up the disk around it to shine brightly, but what powers the jets some of them produce remains a mystery.


What powers a black hole's mighty jets?

Black holes have a reputation for devouring everything in their path. But some of them like to give as well as receive. A small fraction of supermassive black holes—the ginormous ones that lurk at the centers of galaxies—fire off light-speed jets of particles as they snack. A new survey of more than 200 of these cosmic beasts finds that the jets are much more powerful than scientists thought. Astronomers don’t know what powers jets, but this new result, the team says, supports one proposed explanation: The jets are tapping into the rotational energy of the black hole itself.

“It’s very exciting,” says Andrew Fabian, director of the Institute of Astronomy at the University of Cambridge in the United Kingdom, who was not involved in the research. “It’s long been debated whether this is possible.”

About 1% of supermassive black holes have an “accretion disk” of gas and dust swirling around them. When material from this disk falls toward the black hole, the plunging debris gets so hot that it shines more brightly than the whole rest of its galaxy. One in 10 of these active black holes also produces jets that fire out particles at 99.995% of the speed of light. Astrophysicists suspect that accretion disks produce the jets, but they don’t know how.

To get a better idea, a team led by astrophysicist Gabriele Ghisellini of the National Institute for Astrophysics in Merate, Italy, surveyed archival data and picked out a sample of 217 bright supermassive black holes for which they could find gamma ray observations (which reveal the brightness of the jets) and optical observations (to get the luminosity of the accretion disks). Key to the survey were data from NASA’s Fermi Gamma-ray Space Telescope, launched in 2008. “It took time to build up a collection of samples with the required information,”  Ghisellini says.

Plotting the luminosity of the accretion disks against the gamma ray power of their jets, the team reports online today in Nature that there is a clear linear relationship between the two. The brighter the disk, the more powerful the jets—cementing the idea that accretion disks and jets are linked. But in terms of total power being beamed out into space, Ghisellini says, most of the jets were producing 10 times that of their accretion disks. “There must be another engine, not just the gravitational energy [of accreting matter falling toward the black hole].”

The most popular explanation of how jets form is that the fast-spinning accretion disk, which contains charged particles, will produce a powerful magnetic field that is in contact with the black hole. If the black hole is spinning, it drags on the field, winding it into a tight cone at the rotational poles of the black hole. It is this twisted field that accelerates particles away from the black hole as jets and, in the process, extracts energy from the rotation of the black hole. Ghisellini says the group’s finding that jets are so much more powerful than accretion disks shows that disks alone can’t power the jets; the black hole’s spin must also be involved.

Fabian says he still has a “slight reservation” about the assertion that the results prove the role of black hole spin. It’s also possible, he says, that the magnetic field is sucking power out of the accretion disk, making it appear less bright.

“The next step for science is to measure the spin of a black hole,” Ghisellini says, to see whether spin rate is related to jet power. “But it is very hard to measure.” Fabian says researchers using NASA’s NuSTAR x-ray telescope have measured the spin rate of stellar-sized black holes formed from just one or a few stars. Confusingly, some of these small spinning black holes have jets and some don’t. “There must be some other parameter [defining whether a black hole has jets], but we don’t know what that is,” Fabian says.

So although evidence is mounting that black hole spin is powering jets, astrophysicists may have to wait until they can measure the spin of a supermassive black hole before they can nail it. Ghisellini thinks Europe’s Athena x-ray observatory will be able to do the job, but he’s got a long wait ahead: Athena’s launch is slated for 2028.