Research Event Horizon Telescope maps twisting magnetic fields near the supermassive binary black hole candidate OJ287
January 8, 2026
First spatially resolved Event Horizon Telescope observations of OJ 287 reveal shock waves interacting with Kelvin-Helmholtz instabilities in the jet, producing polarization rotations in opposite directions.
For the first time, astronomers have directly observed the dynamic interaction between shock waves (regions of compressed plasma) and helical pressure waves in the jet of a supermassive black hole, thanks to groundbreaking observations by the Event Horizon Telescope (EHT). The results, published today in Astronomy & Astrophysics, offer a stunning visual record of a cosmic "dance" that shapes the powerful beams of energy shooting from the heart of a distant galaxy.
The target of this cosmic spotlight is OJ 287, a supermassive binary black hole system located 1.6 billion light-years away. Using the EHT's extraordinary resolution, equivalent to spotting a ping pong ball on the Moon, the team captured two bright shock waves racing down the jet at different speeds. As these shock waves move downstream through the jet, interacting with the Kelvin-Helmholtz wave pattern within a swirling magnetic field, they produce a remarkable phenomenon: their polarization (the oscillation direction of light waves) rotates in opposite directions.
Opposite Rotations Reveal Jet Physics
The EHT observations captured two shock components propagating downstream at different speeds through a jet threaded by a helical magnetic field. Over the five-day observation period, they exhibit strikingly different polarization behavior: the faster shock shows counterclockwise electric-vector position angle (EVPA) rotations of approximately 3.7 degrees per day, while the slower shock rotates clockwise at approximately 2.5 degrees per day.
Kelvin-Helmholtz instabilities, caused by velocity shear between the jet plasma and surrounding medium, create a helical wave pattern with a wavelength of approximately 100 microarcseconds (1.5 light years). As the shocks propagate through this pattern at different speeds, they illuminate different azimuthal phases of the helical magnetic field. The faster shock sweeps through more rapidly, naturally explaining its faster polarization rotation.
"These opposite-direction rotations are the smoking gun," said Dr. José L. Gómez, lead author from the Instituto de Astrofísica de Andalucía-CSIC. "As the shock components interact with the Kelvin-Helmholtz instability, they illuminate different phases of the helical magnetic field structure, producing the polarization swings we observe."
Shortest Timescale Structural Variability
The observations, taken on April 5 and April 10, 2017, captured the jet at two snapshots separated by just five days, revealing substantial changes in both structure and polarization properties. “We observed substantial changes over five days,” explained Dr. Efthalia Traianou, AGN Working Group Coordinator for the EHT collaboration, one of the lead authors of the paper and a postdoctoral researcher at the IWR research group of Dr. Roman Gold at Heidelberg University. "This is the first time we've directly observed this shock-instability interaction in a black hole jet." The jet exhibits a twisted morphology with rapidly moving components. Detailed modeling revealed that the components move neither in straight ballistic paths from the core nor simply following the curved jet, but rather twisted motions consistent with a bent structure that resembles a helix in projection.
Binary Black Hole Laboratory
OJ 287 is a prime candidate for a binary black hole system, where two supermassive black holes orbit each other; however, alternative scenarios are also considered. This system is known for its dramatic, periodic outbursts, making it a unique laboratory for studying black hole physics. The EHT observations probe the jet at scales of just 10-100 times the primary’s black hole's gravitational radius, right where the jet is launched and where magnetic fields play their most crucial role.
“These measurements let us directly trace the magnetic-field geometry in the jet’s launching and collimation region,” said Dr. Ilje Cho from the Korea Astronomy and Space Science Institute. The observations also revealed a third component approximately 200 microarcseconds downstream with radial polarization consistent with a recollimation shock, a standing feature where the jet re-collimates after initial expansion. The radial magnetic field pattern is characteristic of shock compression observed in other jets.
About the Event Horizon Telescope
More than 300 researchers from Africa, Asia, Europe, North and South America collaborate to operate the Event Horizon Telescope, a global network of radio telescopes that creates a virtual Earth-sized telescope. By linking observatories worldwide through very-long-baseline interferometry (VLBI), the EHT achieves unprecedented angular resolution at millimeter wavelengths. The collaboration previously captured the first images of supermassive black holes: M87* in 2019 and Sagittarius A* at the center of our Milky Way in 2022.
Original publication
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.