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A new observation by the Event Horizon Telescope (EHT), with the participation of the University of Valencia, reveals in detail the turbulent structure of the magnetic field within the jet of material accelerated from a supermassive black hole. This finding provides an unprecedented insight into the physics of supermassive black holes, considered the most powerful “engines” in the universe.
The Event Horizon Telescope (EHT) has just published observations of a relativistic jet – a colossal structure consisting of a collimated beam of plasma strongly accelerated by a supermassive black hole – in which it is possible to observe how internal shock waves interact with the turbulent magnetic field of the jet itself. This discovery, recently published in the journal Astronomy & Astrophysics, offers a rare and close-up view of a rapidly evolving region near a black hole, where relativistic jets are formed and accelerated.
The relativistic jet observed is associated with the central black hole of the blazar OJ 287, located about 1.6 billion light-years away in the constellation Cancer. Blazars are an extreme type of active galaxy whose jet points almost directly towards the Earth, making them one of the most variable and energetic objects in the universe. Using the extraordinary sharpness achievable with the EHT – capable of distinguishing structures the size of a tennis ball on the Moon – the team detected two bright components in the jet that behave like shock waves moving outwards at different speeds. In addition, the light emitted by these shock waves is strongly polarised (polarisation refers to the preference of the light’s electric field to vibrate in a specific direction).
In this case, the polarisation is due to effects related to plasma and to the direction of the magnetic field embedded within the jet. As these bright shock waves travel through the jet, their polarisation changes, making it possible to “scan” the magnetic-field structure as if using a tomographic technique.
The rotation pattern observed in the polarisation of these components – with one rotating in the opposite direction to the other – provides direct evidence that the jet is traversed by a helical magnetic field, with field lines winding along the length of the jet.
Beyond these two shock waves, the images reveal that the jet is not simply straight and smooth. Instead, it displays a twisted, wave-like structure, probably related to so-called Kelvin–Helmholtz instabilities.
As explained by Manel Perucho, professor at the University of Valencia and an expert in relativistic jet dynamics and numerical simulations, “this is a common effect in which differences in velocity between adjacent layers of fluid or gas generate waves and vortices, similar to the ripples and billows that form when two winds interact, but in this case, occurring in a plasma jet moving at speeds close to that of light”.
In addition, José L. Gómez, lead author of the study and researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC), states that “these polarisation rotations in opposite directions are irrefutable proof of our finding. As the shock components propagate along the jet and cross the Kelvin–Helmholtz wave, they illuminate different phases of the helical magnetic field structure, producing the polarisation swings we observe”.
High-precision images of polarised jet emission
Measuring the polarisation of light with the EHT represents a major technical challenge. Polarisation signals are weak and can easily be distorted by small instrumental effects at each telescope. Extracting reliable polarisation maps requires extremely careful calibration and cross-checking throughout the analysis.
“Polarisation is one of the richest sources of information we can measure, but also one of the most fragile”, notes Iván Martí-Vidal, professor at the University of Valencia, a specialist in high-precision VLBI polarimetry and one of the developers of the key calibration techniques used by the EHT. “To be sure that these polarisation rotations actually occur near the black hole and are not caused by instrumental effects, we must model and correct the data with extreme care. The fact that completely independent algorithms recover the same polarisation features from the data provides strong validation of the result”, he explains.
The observations used in the current analysis were made over five days in April 2017 and reveal remarkable changes on an exceptionally short timescale for this source. In just a few days, both the jet structure and its polarised light evolved significantly, demonstrating that the inner jet is a highly dynamic environment.
“We observed substantial changes over five days. With longer and more continuous monitoring, we were able to follow the interaction step by step and build a genuine three-dimensional picture of the jet’s magnetic structure”, explains Efthalia Traianou, from Heidelberg University and the Max Planck Institute for Radio Astronomy.
A cosmic laboratory
The OJ 287 jet has been the scene of spectacular cataclysmic events for more than a century of observations. Astronomers have long debated whether these events could be related to the presence of a second supermassive black hole orbiting the primary one. Whatever the ultimate cause of its long-term variability, OJ 287 remains an important “laboratory” for studying how black holes are fuelled and how their jets respond.
The new EHT images focus on the region where the jet is organised and energised, with a level of sharpness that allows its polarised structure to be resolved directly and its changes to be tracked over time.
New frontiers in jet physics and state-of-the-art simulations
Thanks to their extremely high spatial resolution, EHT observations allow researchers to test physical models of jet behaviour using spatially resolved measurements.
According to José María Martí, professor at the University of Valencia and an expert in relativistic jet physics and numerical relativity, “these observations finally allow us to separate processes in space that previously appeared superimposed”. He adds: “When we combine EHT images with state-of-the-art simulations, we can ask very direct questions: how shock waves propagate, how Kelvin–Helmholtz instabilities shape the flow, or how magnetic fields influence particle acceleration. This is precisely the kind of synergy between data and simulations that we need to understand what makes jets stable, turbulent or radiatively efficient”.
A new window onto the universe
This discovery represents a major step forward in understanding how black holes feed and shape their jets. By resolving the rapid polarisation changes across different jet features, the EHT provides a new way to test ideas about magnetic fields, shock waves, instabilities and particle acceleration in one of the most extreme environments in the known universe.
The result also points towards what may come next: longer and denser time coverage to capture the jet’s evolution not as a series of isolated snapshots, but as a true sequence – a “movie” – revealing in great detail how magnetic structure and plasma dynamics develop.
Article reference: Gómez, J.L., Cho, I., Traianou, E., et al. “Spatially resolved polarization swings in the supermassive binary black hole candidate OJ 287 with first Event Horizon Telescope Observations” Astronomy & Astrophysics, 2026. https://doi.org/10.1051/0004-6361/202555831
Figure description: The polarisation images (shown in orange; left-hand side of the figure) display three bright components that visibly evolve over a five-day interval (5 April on the left; 10 April in the centre). This represents the shortest timescale on which direct images of such changes have been obtained for the jet of this black hole.
The two innermost components exhibit polarisation rotations in opposite directions: the lowest component (labelled “C1/P1”) rotates anticlockwise by approximately 18°, while the central component (labelled “C2/P2”) rotates clockwise by about 12°. The upper, more distant component (labelled “C3*/P3”) shows a radial polarisation structure characteristic of a recollimation shock. On the right-hand side of the figure, a schematic illustrates how shock components (green arrows) propagating at different speeds through the jet interact with a helical pattern of Kelvin–Helmholtz waves (orange lines), sampling different phases of the helical magnetic field (blue lines) and producing the opposite polarisation rotations observed.
