A Radio Movie of the Jet: A New Picture of Plasma Flows Around a Supermassive Black Hole

Twenty-seven years of observations of one of the brightest active galaxies in the Universe have finally come together into a continuous picture. The blazar 3C 345, located at a distance of about five billion light-years, long remained an object of close attention. Its relativistic jet, ejected by the supermassive black hole at its center, displayed bright "knots" that appeared to race at superluminal speed. For decades, astronomers explained them as shock waves — fronts of plasma compression running along the jet faster than the surrounding material. A new paper published in Nature shows that this familiar picture, at least for 3C 345, does not hold up to scrutiny.

The research team applied the kine algorithm — a neural-network method capable of turning scattered radio images into a continuous polarized video. It was based on 116 observations made from 1995 to 2022 by the VLBA network as part of the MOJAVE program at a frequency of 15 GHz. The algorithm processes all the data simultaneously, taking spatiotemporal connections into account, and produces a smooth model that can be "played back" at any moment in time. The result is impressive: the resolution increased by roughly a factor of four compared with traditional methods, and the dynamic range by almost a factor of 140. Instead of individual bright spots, the scientists saw a coherent flow of plasma and were able to measure its local speed at every point along the jet.

The video showed that the bright knots move at approximately the same speed as the surrounding plasma. No noticeable lead, which would be required for shock waves, was found. Moreover, the brightness peaks did not coincide with the polarization maxima — yet another argument against the classical interpretation. The polarization data point rather to locally amplified magnetic regions whose brightness is further increased by the direction of motion. Plasma continuously flows out of the compact core, sometimes ejecting individual bright structures that first fly almost ballistically and then smoothly turn along the jet axis and dissolve into a diffuse "wake."

This discovery so far concerns only 3C 345. The authors emphasize that the conclusions should not automatically be extended to all jets of active galactic nuclei. Nevertheless, the method is already ready for application to archives of hundreds of objects. If a similar picture repeats, ideas about how supermassive black holes accelerate and structure matter could change seriously.

Behind the numbers and algorithms lies something more. Billions of years ago, in a distant galaxy, matter falling onto a black hole gave birth to a narrow relativistic beam that still crosses intergalactic space. We, living on a tiny planet, with the help of radio telescopes scattered across the Earth and neural networks, were able to discern the motion of plasma in this beam. The Universe turns out to be arranged in such a way that its grandest processes — the birth of jets at the event horizon, magnetic fields, relativistic speeds — leave traces available for deciphering. And each new method only strengthens the feeling: the world in which we exist is incredibly precisely and intricately connected, and what happens in its depths far surpasses familiar scales.