Black holes of pretty much any mass can generate jets of material that shoot from their poles. These jets are so incredibly energetic that they can accelerate charged particles to a significant fraction of the speed of light. It’s still not known for sure how the particles in those jets get so much energy and hence move so fast.

One popular hypothesis is the "internal shock" model, which posits that the jets are uneven and lumpy and that some particles are moving faster than others. As a result, those particles will eventually catch up with the slower ones and collide with them. These collisions not only allow energy to be transferred through the jet, but they also allow the jets to have a magnetic field, which can further accelerate charged particles.

A number of observations seem to validate this model, but it has been difficult to observe the process in action and see what makes the jets “tick.” But now, a research team led by Eileen T. Meyer of the Space Telescope Science Institute has gotten a better look at some jets. Using the Hubble Space Telescope’s Advanced Camera for Surveys, the team observed the nearby radio galaxy 3C 264, which is home to a supermassive black hole with impressive jets.

The same jet had been imaged previously a few times over the years, and when the new image was put alongside the earlier ones, it became very apparent that there was a clear progression of events. Objects within the jets, known as knots—essentially denser clusters of charged particles—appear in all four observations (which were taken in 1994, 1996, 2002, and 2014, respectively). Of the four bright blobs, two stayed relatively still within the jet, while the other two moved closer to one another, leading up to a merger seen in the most recent images.

The internal shock model predicts that when two such knots collide, they should brighten significantly, since their individual particles are colliding. In the new observation, the merging knots saw their brightnesses increase by about 40 percent. This bright flash (though the process might be too gradual by a few years to be really considered a flash) provided a clear confirmation of the model.

To better understand the jets, we need to know how efficient the process is. In other words, how much of the jets’ kinetic energy is dissipated and radiated away as light? With the above information in hand, the researchers estimated the amount of lost energy was very low, making this radiative process less efficient than models had predicted (meaning more energy stays in the jet instead of being converted into radiation). But the researchers point out that this is only a lower limit, so the actual value could be higher.

The result will play into understandings of supermassive black holes everywhere in the Universe, which themselves play an important and somewhat mysterious role in the formation and evolution of galaxies. And it may have relevance elsewhere: “This mechanism can explain the variable, high-energy emission from a diverse set of objects,” the authors conclude.

This is one of those science stories in which there’s no immediate end to the answers. Finding out in detail how much energy 3C 264's jet is losing, both to radiation and to the generation of its magnetic field, will require researchers to continue monitoring the jet for decades to come.

Nature, 2015. DOI: 10.1038/nature14481 (About DOIs)