A new study involving ICCUB researcher Valentí Bosch-Ramon, published last week in Nature Astronomy, has uncovered how collisions between blobs of matter constituting black hole jets can drastically reshape these outflows' magnetic field. 
 

An International team of researchers have uncovered a new piece of the puzzle behind the enigmatic jets of SS 433, one of the best-studied microquasars in our Galaxy. Using advanced three-dimensional magnetohydrodynamics simulations, the team has demonstrated that collisions between individual plasma blobs making up the jets in this system can naturally reorganize the magnetic field, reproducing observational features that have puzzled astronomers for years.

Blackhole jets and SS 433

Black holes are known for swallowing matter, but some of them also produce powerful jets that expel material back into space at enormous speeds. These jets can extend across vast distances and influence their surroundings, yet many aspects of how they work remain poorly understood. Now, astronomers have shed new light on one of the most unusual jet-producing systems in the Milky Way: SS 433. Their results identify a physical mechanism that can explain how the magnetic fields inside SS 433 jets evolve as they travel away from the black hole. 
 

SS 433 is a binary system located about 18,000 light-years from Earth. It consists of a massive star orbiting a compact object that is most likely a stellar-mass black hole. As the black hole pulls material from its companion, part of that material is accelerated into two opposite jets moving at about 26% of the speed of light. The jets of SS 433 also precess, meaning that their direction slowly changes over time, causing them to trace a giant corkscrew-shaped pattern through space.

The hidden collisions shaping magnetic fields

For decades, observations of SS 433 have shown a puzzling behaviour. Near the source, the magnetic field appears to follow the helical, or corkscrew-like, shape of the jets. At larger distances, however, it becomes aligned with the direction in which the jet material is moving, that is, radially away from source center. Astronomers had proposed various explanations for this transition, but no theory had yet demonstrated in a self-consistent way how it could emerge from the jet's internal dynamics. 
 

To investigate the problem, researchers performed three-dimensional numerical simulations that reproduce the behaviour of a relativistic magnetised plasma, a hot gas made of electrically charged particles. The simulations compared two different scenarios. In the first, the jet behaves as a continuous flow. In the second, it is composed of discrete ejecta: individual blobs of plasma launched one after another at slightly different speeds, as observations of SS 433 suggest. 
 

The results reveal that the magnetic field behaves very differently in the two cases. In a continuous jet, the magnetic field largely preserves its original structure and continues to follow the corkscrew-shaped trajectory of the flow. But when the jet consists of discrete ejecta, faster blobs catch up with slower ones launched earlier. These collisions generate internal shock waves, regions where matter is abruptly compressed, much like a traffic jam forms when faster cars encounter slower traffic ahead. 
 

According to the simulations, these collisions compress, stretch and reorganise the magnetic field lines. As a result, the field gradually loses its alignment with the corkscrew path of the jet and instead becomes aligned with the radial direction in which the plasma blobs are moving. This reproduces remarkably well the magnetic-field orientations measured in radio observations of SS 433. 
 

The study also found that the collisions do not destroy the jets. On the contrary, they merge smaller ejecta into larger, elongated structures that are more stable and can travel greater distances without breaking apart. This suggests that interactions between ejecta may play an important role not only in shaping magnetic fields but also in determining how jets evolve over time on large scales. 
 

Valentí Bosch-Ramon, researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and co-author of the study, highlights: "these simulations demonstrate how microquasar jets interacting with their environments can reshape the plasma properties in unexpected ways; they are thus a powerful tool that in combination with observations can probe the jet structure in ways not directly accessible by these observations alone." 
 

Although the simulations were specifically designed to reproduce the properties of SS 433, the researchers believe that the same mechanism could operate in other systems that produce precessing jets. The work therefore provides a new framework for interpreting observations of black holes and other compact objects, while offering fresh clues about the role of magnetic fields in some of the Universe's most extreme environments.

Reference:

José López-Miralles, Manel Perucho, David Vallés-Pérez, José-María Martí, Valentí Bosch-Ramon, James C. A. Miller-Jones, Sara E. Motta, Simone Migliari & Herman L. Marshall. Magnetic field topology and colliding discrete ejecta in the precessing jets of SS 433. Nature Astronomy (2026). DOI: 10.1038/s41550-026-02922-6.