Astronomers watch 2 supermassive black holes caught in a twisted dance with never-before-seen jet behavior
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Images of the twisted jet of matter OJ287 emerging from a distant black hole seen by the EHT. | Credit: EHT Collaboration / E. Traianou
Astronomers used the Event Horizon Telescope (EHT) to observe a violent cosmic dance between a suspected pair of supermassive black holes at the heart of a distant galaxy. The evidence for this rendezvous between cosmic monsters lies in the twisted properties of the jets that erupt around black holes.
THE supermassive black hole A pair, or binary, lurks at the heart of quasar OJ287, located at the center of a galaxy about 1.6 billion light-years from Earth. Using a level of resolution capable of spotting a tennis ball on the Moon’s surface, the team spotted two shock waves flowing into OJ287’s jet. Interestingly, the shocks were observed moving at different speeds. And as they travel, crossing powerful magnetic fields, these shock waves seem to produce a phenomenon never seen before.
This is just the latest major breakthrough in black holes by the EHT, which in April 2017 captured the first-ever image of a black hole, the supermassive black hole. M87*made public in 2019. The telescope array followed with an image of Sagittarius A*, THE Milky Way‘s own supermassive black hole, which the public could see in 2022.
Since then, EHT has continued to make waves in black hole science.
“This result shows that EHT is not only useful for producing spectacular images, but can also be used to understand the physics that governs black hole jets,” EHT team member Mariafelicia De Laurentis said in a statement. “The observational distinction between what is due to geometry and what is instead the result of real physical processes is a key step in comparing theoretical models with observations.”
Snapshots of a black hole jet
The team captured two snapshots of the OJ287 system on April 5, 2017, and again on April 10 of the same year. These revealed substantial changes in the structure and polarization of OJ287 occurring over the course of just five Earth days. This is the shortest interval during which such changes have been observed in a black hole jet.
These changes are thought to be the result of shocks interacting with velocity instabilities called Kelvin-Helmholtz instabilities. They result in a highly twisted structure within a jet, with three distinct polarized components: two slower and rotating in opposite directions relative to each other, one faster and rotating counterclockwise. This represented the first direct confirmation of a helical magnetic field with a black hole jet.
“We spatially resolve the individual components of the shock and observe their interaction with Kelvin-Helmholtz instabilities,” said team member Ilje Cho from the Korea Institute of Astronomy and Space Science. “This is the first time we have directly observed this interaction between shocks and instabilities in a black hole jet.”
A diagram showing the helical structure of instabilities in the OJ287 jet | Credit: EHT Collaboration / E. Traianou
“These observed variations in the jet are usually interpreted in terms of a precession effect of the jet itself. However, precession models would expect the jet components to move ballistically along the jet,” said EHT team member Rocco Lico from the Italian National Institute of Astrophysics (INAF). “Our observations, however, indicate non-ballistic movements of these components, calling into question the precession hypothesis as the sole explanation for the observed source morphology.” The rapid motions measured by the team suggest that the kinetic energy of the particles exceeds the magnetic energy in the inner regions of the jet. This favors the development of Kelvin–Helmholtz instabilities, due to the difference in speed at the surface between the jet, which moves at speeds close to the speed of light, and the surrounding matter, which is much slower. These instabilities can cause helix-shaped distortions that manifest as a “twisted” structure, just like the one the EHT spotted in the OJ287 jet.
The twisted structure of the jet observed in OJ287, the high degree of polarization of the three components and the evolution of their polarization angles indicate a complex interaction between Kelvin–Helmholtz instabilities and shocks in a jet imbued with a helical magnetic field.
“These rotations in opposite directions are irrefutable proof,” research team leader José L. Gómez of the Instituto de Astrofísica de Andalucía-Csic said in the release. “When the components of the shock wave interact with the Kelvin-Helmholtz instability, they illuminate different phases of the helical structure of the magnetic field, producing the polarization oscillations we observe.”
A GIF showing how the polarization of OJ287’s jet has changed over time | Credit: EHT/E. Traianou collaboration.
The team’s model proposes that Kelvin-Helmholtz instabilities generate filamentous structures that interact with shock propagation in the jet.
“These interactions compress the magnetic field and amplify the emission in specific regions of the jet, explaining the features observed in both total intensity and polarized light, as well as the rapid variations in polarization angles and apparent non-ballistic movements observed, despite the presence of a generally straight jet,” Lico said. “For the first time, high-resolution EHT data allows us to directly visualize these structures, providing concrete evidence of the interaction between jet instabilities, shocks and helical magnetic fields.”
OJ287 was the ideal candidate to make these observations because the dancing supermassive black holes in this pair are well known for their periodic explosions, making it a unique laboratory for studying black hole physics.
The team’s research was published January 8 in the journal Astronomy and astrophysics.
