Cambridge, MA -- In 2019, the EHT released its first image of a ring of hot plasma close to the event horizon of M87*. In 2021, EHT produced new images in polarized light, giving the first sign that the magnetic fields close to the black hole were strong. The measurements reported today demonstrate the presence of spiraling light - or circular polarization - providing more conclusive evidence for these strong magnetic fields.
One paper was led by Freek Roelofs, a postdoctoral fellow at the Smithsonian Astrophysical Observatory (SAO), a part of the Center for Astrophysics | Harvard & Smithsonian (CfA). Roelofs' analysis revealed a surprising asymmetry across the image, which is only visible in spiraling light and was not seen in other methods that made weaker assumptions. "Working together to see what we can and cannot squeeze out of our data has made this project incredibly exciting and rewarding," Roelofs said.
"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," adds former CfA member and EHT polarization project coordinator Andrew Chael, who is currently an associate research scholar at the Gravity Initiative at Princeton University.
"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."
The team used these observations to test different theories for the motion of plasma and magnetic field around the black hole, including a set of state-of-the art supercomputer simulations. "The circular polarization observations bolster our confidence 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 CfA and a Fellow at the Harvard Black Hole Initiative. 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."
"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."
