Black hole jets, how do they work? Magnets! | Ars Technica

Well, a really intense magnetic field, at least.

A radio image of a quasar, taken by the Very Large Array. The white dot in the middle is the core, while the protrusions pointed top-left and bottom-right are jets, traveling at relativistic speeds, culminating in lobes.
Image courtesy of NRAO/AUI

At the centers of some massive galaxies, supermassive black holes power incredibly bright objects called quasars. Black holes gobble up matter so quickly that the infalling matter heats up from friction and emits light. While this disk of accreting matter is incredibly bright on its own, the black hole has another source of light: jets erupt from the poles of the black hole, shooting particles at speeds approaching that of light. These jets are incredibly bright—possibly brighter than the accretion disk.

It’s not known for sure what causes the jets. It’s thought that the black hole’s spin and mass interact with the magnetic field near the black hole to accelerate the particles. While some evidence supports this model, it’s been difficult to test, mainly because scientists lacked a full knowledge of how bright the accretion disks is. But a new study of a sample of blazars (quasars with jets that point toward Earth) shows a clear correlation between the jets’ power and the accretion disk’s brightness. This suggests that the magnetic field is a factor in producing the jets.

The researchers examined 217 blazars using data obtained by the Fermi observatory, looking for some relationship between the jets’ power and the accretion disk. Blazars are useful because with a blazar, we get direct light from both the accretion disk and the jet, since the latter is pointed toward us. And we can tell which is which, because light from the jet is mostly in the form of gamma rays, while the accretion disk produces a broader emission spectrum.

It turns out that when the luminosity of the jets are graphed against the luminosity of the disk, there’s a clear, straight-line relationship, with the jet having more power than the disk’s luminosity.

This is consistent with theoretical predictions. The disk’s luminosity is controlled by the rate at which the black hole is consuming the disk material—the more matter there is, the faster it falls in, the hotter it gets, and the more light it produces. Since the disk also has a magnetic field that scales with the amount of matter present, an increased amount of matter in the disk increases the power of the jet—the accretion disk really does have a magnetic influence on the jets.

The results also suggest that the jets are incredibly efficient. “The process that launches and accelerates jets must be extremely efficient,” the authors write in their paper. “And might be the most efficient way of transporting energy from the vicinity of the black hole to infinity.”

Of course, this study covers a very specific kind of quasar, leaving plenty of future work needed to understand the full mechanism behind the jets.

“Our source sample consists by construction of luminous [gamma]-ray sources that presumably have the most powerful jets, and thus have the most rapidly spinning holes,” they write. “It will be interesting to explore less luminous jetted sources, to gain insight into the possible dependence of the jet power on the black hole spin and the possible existence of a minimum spin value for the jet to exist.”

This, in turn, could shed some light, so to speak, on why the jets form in the first place.

Nature, 2014. DOI: doi:10.1038/nature13856 (About DOIs).

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