A supermassive black hole’s strong magnetic fields are revealed in a new light

New M87* results from the Event Horizon Telescope

November 08, 2023

The Event Horizon Telescope (EHT) collaboration, with major contributions from the Max-Planck-Institut für Radioastronomie (MPIfR) and the Institut de Radioastronomie Millimétrique, has published new results describing for the first time how light from the edge of the supermassive black hole M87* spirals as it escapes the black hole's intense gravity. This signature, known as circular polarisation, is a consequence of the rotation of the electric field in the radio waves. As it travels, it carries information about the magnetic field and types of energetic particles around the black hole. The new work, published today in Astrophysical Journal Letters, supports earlier findings from the EHT that the magnetic field around the M87* black hole is strong enough to occasionally prevent the black hole from swallowing nearby matter.

“Circular polarization is the final signal we looked for in the EHT’s first observations of the M87 black hole, and it was by far the hardest to analyze,” says  Andrew Chael, an associate research scholar at the Gravity Initiative at Princeton University, who coordinated the project presented today. “These new results give us confidence that our picture of a strong magnetic field permeating the hot gas surrounding the black hole is the right one. The unprecedented EHT observations are allowing us to answer long-standing questions about how black holes consume matter and launch jets outside their host galaxies."

In 2019, the Event Horizon Telescope (EHT) reached a milestone when it unveiled its first image, showing a glowing ring of superheated plasma close to the event horizon of the enigmatic M87* black hole. Then, in 2021, EHT scientists unveiled an image showing the orientations of the light’s electric fields within the image. This particular aspect of the image, known as linear polarisation, served as the first clue to the presence of well-ordered and strong magnetic fields in the vicinity of the black hole. The observing array includes, among others, the APEX radio telescope in Chile, built by the MPIfR.

“Building on this, our new measurements of circular polarisation, which reveal the way in which the electric fields of light spiral around the linear direction identified in the 2021 analysis, provide even more compelling confirmation of the existence of these powerful magnetic fields.” says Eduardo Ros, researcher at the MPIfR, and co-author of the published work.

“The signal in circular polarization is about 100 times weaker than the unpolarized data we used to make the first black hole image,” says Ioannis Myserlis, a staff astronomer at the Institut de Radioastronomie Millimétrique (IRAM). “Finding this  weak signal in the data was like trying to listen to a conversation next to a jackhammer. We had to carefully test our methods to determine what we could really trust.”

To perform this careful analysis, the team created several new methods for reconstructing a polarized image from the EHT’s sparse, noisy measurements and tested them thoroughly. “Testing our different analysis methods against simulated data and against each other was critical,” says Freek Roelofs, a postdoctoral fellow at the Center for Astrophysics | Harvard and Smithsonian. In a companion paper, also published today, Roelofs found that when he enforced some assumptions about the image in his analysis, the data revealed a surprising difference in the left- and right-handed light across the ring we see. However, other methods that made weaker assumptions did not see this difference. “Working together to see what we can and cannot squeeze out of our data has made this project incredibly exciting and rewarding,” Roelofs comments.

The team performed different tests on the data, and all of them found unambiguous evidence that circularly polarized light exists close to the event horizon. Maciek Wielgus, researcher at the MPIfR and member of the team, says: “In the end, because the EHT’s measurements of circular polarization were very weak, our team was not yet able to come up with a more detailed image of the ‘handedness’ of the spiraling light. Instead, we were able to determine that the circularly polarized or spiraling part of the light could only be a small fraction of the total light making up the black hole image.”

In a recent study, the EHT team utilized a specific measurement technique to investigate various hypotheses regarding the shape and behavior of plasma and magnetic fields surrounding a black hole. This investigation included the use of cutting-edge supercomputer simulations. Notably, the observations of circular polarization have provided additional support to prior findings that suggest the presence of powerful magnetic fields. These magnetic fields exert a significant force, pushing against matter falling into the black hole and facilitating the emergence of robust plasma jets that extend far away from the central region of the M87 galaxy.

The collective analysis of both simulations and observations unveils a tumultuous and dynamic setting just beyond the black hole's event horizon. In this region, magnetic fields, gravity, and superheated plasma interact violently.

“While  the EHT data from 2017 may not possess the requisite sensitivity to unveil the  detailed structure of circular polarization around the black hole, we remain optimistic about the future.” says Thomas Krichbaum from the MPIfR, one of the  pioneers in mm-VLBI. “Our ongoing analysis of more recent EHT datasets holds the promise of yielding more precise detections of this signal. This then would shed light on whether matter-antimatter pairs of particles contribute to the particle composition of the plasma near the event horizon and elucidate the mechanisms behind their acceleration to speeds approaching that of light”, he concludes.

"Working with these groundbreaking observations has undoubtedly been a formidable challenge, but it has prepared us for the exciting prospects ahead," adds Anton Zensus, founding board chair of the EHT collaboration and director at the MPIfR.  He comments: "The EHT is currently undergoing rapid expansion, with new telescopes and improved technologies at all its observatory sites, and relying on our VLBI correlator in Bonn".

More Information

The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, and North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems, creating a fundamentally new instrument with the highest angular resolving power that has yet been achieved.

The individual telescopes involved are: ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope, and the Greenland Telescope (GLT).

The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University, and the Smithsonian Astrophysical Observatory.

Following scientists affiliated to the MPIfR are co-authors of the publication:
Walter Alef, Rebecca Azulay, Uwe Bach, Anne-Kathrin Baczko, Silke Britzen, Gregory Desvignes, Sergio A. Dzib, Ralph P. Eatough, Christian M. Fromm, Michael Janssen, Dong-Jin Kim, Jae-Young Kim, Joana A. Kramer, Michael Kramer, Thomas P. Krichbaum, Mikhail Lisakov, Jun Liu, Kuo Liu, RuSen Lu, Andrei P. Lobanov, Nicholas R. MacDonald, Nichola Marchilli, Karl M. Menten, Cornelia Müller, Hendrik Müller, Aristeidis Noutsos, Gisela N. Ortiz-León, Georgios F. Paraschos, Felix M. Pötzl, Eduardo Ros, Helge Rottmann, Alan L. Roy, Tuomas Savolainen, Lijing Shao, Pablo Torne, Efthalia Traianou, Jan Wagner, Robert Wharton, Maciek Wielgus, Gunther Witzel, and J. Anton Zensus.

IRAM, both at the sites in Granada and Grenoble, has  the following affiliated scientists as members of the team who presented today’s results: Michael Bremer, Dominique Broguiere, Sergio A. Dzib, Roberto García, Olivier Gentaz, Carsten Kramer, Ioannis Myserlis, Roberto Neri, Vincent Piétu, Ignacio Ruiz, Salvador Sánchez, Miguel Sánchez-Portal, Karl-Friedrich Schuster, and Pablo Torne.

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