Unexpected polarization reversal in black hole M87* tests theoretical models

The Event Horizon Telescope (EHT) collaboration, which includes researchers from the National Institute of Astrophysics (INAF), the National Institute of Nuclear Physics (INFN) and the Federico II University, has presented new, detailed images of the supermassive black hole at the center of the galaxy M87 - known as M87* - revealing a dynamical environment, with varying polarization configurations near the black hole. Also, for the first time, signals of the extended jet emission near its base, which is connected to the ring around M87*, were detected. These new observations, published in Astronomy & Astrophysics, provide new insights into how matter and energy behave in the extreme environments surrounding black holes.

"These results show how the EHT is evolving into a full-fledged scientific observatory, capable not only of producing unprecedented images, but also of building a progressive and coherent understanding of black hole physics," said Mariafelicia De Laurentis, EHT project scientist, INFN researcher and professor at the University Federico II. "Each new campaign broadens our horizon, from plasma dynamics and magnetic fields to the role of black holes in cosmic evolution. It is a concrete demonstration of the enormous scientific potential of this instrument."

The first image of M87*, located about 55 million light-years from Earth and with a mass more than six billion times the mass of the Sun, dates back to 2019 and is due to EHT's global network of radio telescopes, which act together as a single observatory the size of Earth. Now, thanks to observations in 2017, 2018 and 2021, the collaboration has taken another step toward understanding how magnetic fields near the black hole change over time.

"What's remarkable is that while the size of the ring has remained constant over the years-confirming the shadow of the black hole predicted by Einstein's theory-the polarization pattern changes significantly," explained Paul Tiede, an astronomer at the Center for Astrophysics | Harvard & Smithsonian and co-responsible for the study. "This tells us that the magnetized plasma swirling near the event horizon is far from static: it is dynamic and complex, challenging our theoretical models."

Between 2017 and 2021, the polarization pattern reversed direction. In 2017, the magnetic fields appeared to wrap in one direction; in 2018, they had stabilized; and in 2021, they reversed, wrapping in the opposite direction. Some of these observed changes in the direction of polarization rotation could be influenced not only by the internal magnetic structure, but also by external effects, such as the presence of a magnetized plasma that acts as a "Faraday screen"-a "veil" of magnetized gas that alters the light signal before it reaches our telescopes-altering the polarization along the line of sight. The cumulative effects of changing polarization over time suggest an evolving turbulent environment in which magnetic fields play a crucial role in regulating how matter falls into the black hole and how energy is expelled outward.

"Again, to ensure the robustness of the results, we used several image reconstruction techniques that were completely independent of each other," said Rocco Lico, INAF researcher and information technology officer at the EHT. "To reach these new milestones, it was also necessary to develop new analysis tools, which makes the work even more exciting."

"Year after year, we improve the EHT with additional telescopes and updated instrumentation, new ideas for science explorations, and innovative algorithms to get more out of the data," added Michael Janssen, associate professor at Radboud University in Nijmegen and a member of the EHT science board. "For this study, all these factors combined perfectly, leading to new scientific results and new questions, which will surely keep us busy for many years to come."

Crucial in the 2021 EHT observations were two new telescopes-Kitt Peak in Arizona and NOEMA in France-that increased the sensitivity and sharpness of the images. This allowed scientists to constrain, for the first time with EHT, the direction of the emission at the base of the relativistic jet of M87*: a narrow beam of energetic particles escaping from the black hole at close to the speed of light. Jets like that of M87* play a key role in the evolution of galaxies, regulating star formation and distributing energy over vast scales. Its emission across the electromagnetic spectrum-including gamma rays and neutrinos-provides a unique laboratory for studying how these cosmic phenomena form and are generated.