New EHT Images Reveal Unexpected Polarization Flips at M87*

Between 2017 and 2021, the polarization pattern flipped direction. In 2017, the magnetic fields appeared to spiral one way; by 2018, they settled; and in 2021, they reversed, spiraling the opposite direction. Some of these apparent changes in the polarization’s rotational direction may be influenced by a combination of internal magnetic structure and external effects, such as a Faraday screen. The cumulative effects of how this polarization changes over time suggests an evolving, turbulent environment where magnetic fields play a vital role in governing how matter falls into the black hole and how energy is launched outward. 

'The fact that the polarization pattern flipped direction from 2017 to 2021 was totally unexpected', Paul Tiede explains. 'It challenges our models and shows there’s much we still don’t understand near the event horizon.'

Sera Markoff of the University of Amsterdam adds: 'Thanks to the work of Dutch scientists, we saw a gamma-ray burst at the same time as the polarisation rotation. We know that magnetic fields play a major role in accelerating particles that can lead to such a burst, so it is possible that the rotation is connected to this burst.' 

New Telescopes

Crucially, the 2021 EHT observations included two new telescopes—Kitt Peak in Arizona and NOEMA in France—which enhanced the array’s sensitivity and image clarity. This allowed scientists to constrain, for the first time with the EHT, the emission direction of the base of M87’s relativistic jet—a narrow beam of energetic particles blasting out from the black hole at nearly the speed of light. 

'The improved calibration has led to a remarkable boost in data quality and array performance, with new short baselines— between NOEMA and the IRAM 30m telescopes, and between Kitt Peak and SMT, providing the first constraints on the faint jet base emission', said Sebastiano von Fellenberg, a postdoctoral fellow at the University of Toronto’s Canadian Institute for Theoretical Astrophysics (CITA), and postdoctoral researcher at the Max Planck Institute for Radio Astronomy (MPIfR) who focused on calibration for the project. 'This leap in sensitivity also enhances our ability to detect subtle polarization signals.'

Unique laboratory

Jets like M87’s play a crucial role in galaxy evolution by regulating star formation and distributing energy on vast scales. Emitting across the electromagnetic spectrum—including gamma rays and neutrinos—M87’s powerful jet provides a unique laboratory to study how these cosmic phenomena form and are launched. This new detection offers a vital piece of the puzzle.

The EHT is an international collaboration of scientists that creates images of black holes. Astronomers link telescopes around the world to create a large virtual telescope the size of the Earth. In 2019, the Event Horizon Telescope published the first photo of a black hole. Astronomers from Radboud University, the University of Amsterdam, Leiden University, the NOVA technical submm group of the University of Groningen and JIVE are involved in the project. Dutch scientists have made essential contributions to this research. Michael Janssen from Radboud University led the team together with Paul Tiede (Harvard Smithsonian). Monika Moscibrodzka (Radboud University) led the first analysis of the polarisation data from 2017 in 2021. Sera Markoff (University of Amsterdam) is co-coordinator of the group looking at the radiation from the source at other wavelengths, which is important for interpreting the new results.

The NOEMA measurements and contributions from the Netherlands to the observations were previously made possible by contributions from Radboud and the ERC. The next step in black hole research is the expansion of the EHT with the Africa Millimetre Telescope in Namibia, which will be built in the coming years thanks to grants from the ERC, NWO and a guarantee from Radboud University. This telescope will form a crucial link between telescopes in Europe, South and Latin America and at the South Pole, enabling colour films of black holes to be made in the future.

The ERC Synergy Grants ‘BlackHoleCam’ by Heino Falcke (Radboud), Michael Kramer (MPIfR Bonn) and Luciano Rezzolla (University of Frankfurt) and ‘Blackholistic’ by Heino Falcke, Sera Markoff and Rob Fender (Oxford) have been of great importance to this research.