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

The Event Horizon Telescope (EHT) collaboration has published new results that describe 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, a signature known as circular polarization. The way light’s electric field prefers to rotate clockwise or counterclockwise as it travels carries information about the magnetic field and types of high-energy particles around the black hole. The new paper, published today in Astrophysical Journal Letters,  supports earlier findings from the EHT that the magnetic field near the M87* black hole is strong enough to occasionally stop the black hole from swallowing up 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.  “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 EHT released its first image of a ring of hot plasma close to the event horizon of M87*.  In 2021, EHT scientists released an image showing the directions of the oscillating electric fields across the image. Known as linear polarization, this result was the first sign that the magnetic fields close to the black hole were ordered and strong. The new measurements of the circular polarization – which indicate how light’s electric fields spiral around the linear direction from the 2021 analysis – provide yet more conclusive evidence for these strong magnetic fields. 

“The signal in circular polarization is 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,” said 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 strong 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 said.

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. In the end, because the EHT’s measurements of circular polarization were much weaker than the earlier datasets, the team was unable to come up with an unambiguous image of the  ‘handedness’ of the spiraling light. Instead, the team was 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.      

The team applied this measurement to test different theories for the shape and movement of the plasma and magnetic field around the black hole, including a set of state-of-the art supercomputer simulations. “The circular polarization observations bolster our earlier results indicating that the magnetic fields are strong enough to push back on the infalling matter and help launch the strong jets of plasma we see extending throughout the M87 galaxy,” said Angelo Ricarte, a postdoctoral fellow at the Black Hole Initiative at Harvard University. Abhishek Joshi, a graduate student at the University of Illinois, added, “It’s wonderful to directly compare our simulations to these cutting-edge observations. Together, they paint a picture of a chaotic, violent environment just outside the event horizon, where magnetic fields, gravity, and hot plasma strongly interact with each other.”

Although the existing EHT data are not sensitive enough to determine the structure of the black hole’s circular polarization, the team has high hopes for the future. Ongoing analysis of more recent EHT datasets could yield better  detections of this signal, which could answer further questions about whether matter-antimatter pairs of particles make up any of plasma outside the event horizon and how they are accelerated to velocities close to the speed of light.

“Working with these pioneering observations was really challenging, but it trained us well for the future,” said Svetlana Jorstad, a senior research scientist at Boston University. "The EHT is growing rapidly, with new telescopes and better technology at all our sites, giving us a more sensitive and detailed set of data to work with. It’s exciting to contemplate how much better our results could get in just a short time.”