Feature - Black holes and their jets
Jets of particles streaming from black holes in far-away galaxies operate differently than previously thought, according to a study published recently in Nature.
High above the flat Milky Way galaxy, bright galaxies called blazars dominate the gamma-ray sky, discrete spots on the dark backdrop of the universe. As nearby matter falls into the black hole at the center of a blazar, "feeding" the black hole, it sprays some of this energy back out into the universe as a jet of particles.
"As the universe's biggest accelerators, blazar jets are important to understand," said Masaaki Hayashida, a corresponding author on the Nature paper and a research fellow with the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). "But how they are produced and how they are structured is not well understood. We're still looking to understand the basics."
Researchers had previously theorized that such jets are held together by strong magnetic field tendrils, while the jet's light is created by particles revolving around these wisp-thin magnetic field "lines." Until now, scientists were forced to formulate computationally-intensive simulations of models, such as those pictured above and below, based on inadequate data. The recent study, which included data from more than 20 telescopes worldwide, constitutes a great leap towards changing that.
"This work is a significant step toward understanding the physics of these jets," said KIPAC Director Roger Blandford. "It's this type of observation that is going to make it possible for us to figure out their anatomy."
Locating the gamma rays
Over a full year of observations, the researchers focused on one particular blazar jet, located in the constellation Virgo, monitoring it in many different wavelengths of light: gamma-ray, X-ray, optical, infrared and radio. Blazars continuously flicker, and researchers expected continual changes in all types of light. Midway through the year, however, researchers observed a spectacular change in the jet's optical and gamma-ray emission: a 20-day-long flare in gamma rays was accompanied by a dramatic change in the jet's optical light.
Although most optical light is unpolarized-consisting of light rays with an equal mix of all polarizations or directionality-the extreme bending of energetic particles around a magnetic field line can polarize light. During the 20-day gamma-ray flare, optical light streaming from the jet changed its polarization. This temporal connection between changes in the gamma-ray light and changes in the optical light could mean that both types of light are created in the same geographical region of the jet; during those 20 days, something in the local environment altered to cause both the optical and gamma-ray light to vary.
"We have a fairly good idea of where in the jet optical light is created; now that we know the gamma rays and optical light are created in the same place, we can for the first time determine where the gamma rays come from," said Hayashida.
The great majority of energy released in a jet escapes in the form of gamma rays, and researchers previously thought that all of this energy must be released near the black hole, close to where the matter flowing into the black hole gives up its energy in the first place. Yet optical light is emitted relatively far from the black hole. If Hayashida is right, then the same must be true of the gamma rays. And this, Hayashida and his co-author Greg Madejski said, in turn suggests that the magnetic field lines must somehow help the energy travel far from the black hole before it is released in the form of gamma rays.
"What we found was very different from what we were expecting," said Madejski. "The data suggest that gamma rays are produced not one or two light days from the black hole [as was expected] but closer to one light year. That's surprising."
As exciting as that might be, there are other explanations for the results Hayashidi and Madejski observed, according to Jonathan McKinney, a theoretical astrophysicist from Stanford University.
"The jet can sort of wobble around. They assume that the wobble was always very regular, and that's how they prove their point about how the gamma rays are generated far away," McKinney explained. "All you have to do is make the wobble very random to show that the gamma rays can come from very close to the black hole."
Rethinking jet structure
The gradual change of the optical light's polarization may reveal something unexpected about the overall shape of the jet: the jet appears to curve as it travels away from the black hole.
"At one point during a gamma-ray flare, the polarization rotated about 180 degrees as the intensity of the light changed," said Hayashida. "This suggests that the whole jet curves."
This new understanding of the inner workings and construction of a blazar jet requires a new working model of the jet's structure, one in which the jet curves dramatically and the most energetic light originates far from the black hole. This, Madejski said, is where theorists come in. "Our study poses a very important challenge to theorists: how would you construct a jet that could potentially be carrying energy so far from the black hole? And how could we then detect that? Taking the magnetic field lines into account is not simple. Related calculations are difficult to do analytically, and must be solved with extremely complex numerical schemes."
McKinney, whose own research focuses on the formation of magnetized jets, agrees that the results pose as many questions as they answer. "There's been a long-time controversy about these jets-about exactly where the gamma-ray emission is coming from. This work constrains the types of jet models that are possible," said McKinney, who is unassociated with the recent study. "From a theoretician's point of view, I'm excited because it means we need to rethink our models."
Portions of this article originally appeared in a SLAC press release.
-Kelen Tuttle, Symmetry Magazine, and Miriam Boon, iSGTW