The text discusses the ongoing progress in understanding supermassive black holes that are accreting matter and emitting particle jets, particularly in the form of quasars. The Event Horizon Telescope (EHT) collaboration has been actively working on unraveling phenomena involving the rotation of black holes and powerful magnetic fields. The recent advancement involves the detection of circularly polarized light emissions.
In 1964, Russian astrophysicists Yakov Zel’dovich and Igor Novikov, along with Edwin Salpeter in the United States, proposed independently that quasars and active galactic nuclei are supermassive black holes accreting matter. Donald Lynden-Bell and Martin Rees even suggested one at the center of the Milky Way in 1971. Recent discoveries by the Event Horizon Telescope collaboration, using a network of radio telescopes on a global scale, have provided images showing the shadow of the event horizons of the black holes M87* and Sgr A*.
M87* serves as a laboratory for testing ideas about rotating Kerr black holes, the accretion disks surrounding them, and the electromagnetism and magnetohydrodynamics of plasmas responsible for emitting relativistic particle jets. These findings are crucial for understanding quasars and their roles in galaxy evolution.
Researchers have been studying the magnetic fields around M87*, focusing on the phenomenon of light polarization for some time now.
Circular Polarization of Light
Circular polarization of light is a phenomenon that arises when the electric field of a light wave rotates in a circular manner as it propagates. To grasp this concept, let’s revisit the explanation provided by Futura in a previous article and draw upon insights from Feynman’s teachings. A light wave can be described as an electric field represented by an arrow perpendicular to the direction of the associated light ray’s propagation. This arrow oscillates, akin to a weight at the end of a spring. Linear polarization occurs when the arrow’s direction remains along a straight line parallel to the wave’s propagation. Conversely, non-polarized light would have an arrow whose direction chaotically varies, although always perpendicular to the light ray. Circular polarization occurs when the arrow oscillates while regularly rotating its tip around a circle.
Light emitted by a heated body, such as the Sun, is typically unpolarized. However, in the presence of a magnetic field or when interacting with certain materials—such as reflecting off glass or passing through a crystal like Iceland spar—light can become polarized. Consequently, measuring the polarization of a light wave provides valuable information about the magnetic field of celestial bodies, like the Sun, or the structure and properties of the traversed matter.
Recently, two groups of researchers have identified another state of polarization in the light emanating from the environment of M87* and its associated magnetic fields. They have announced the discovery of circularly polarized radiation.
In an article led by Freek Roelofs, a postdoctoral researcher at the Smithsonian Astrophysical Observatory (SAO), affiliated with the Center for Astrophysics | Harvard and Smithsonian (CfA), and another article by members of the Event Horizon Telescope Collaboration, these findings have been published in The Astrophysical Journal Letters.
Several statements from the researchers involved in this discovery and its implications are included in a press release from the Center for Astrophysics | Harvard and Smithsonian (CfA).
Turbulent and Chaotic Plasmas
Andrew Chael, former CfA member and project coordinator for the EHT polarization project—now a researcher associated with the Gravity Initiative at Princeton University—explains, “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.“
His colleague, Ioannis Myserlis, an astronomer at the Institute of Millimeter Radio Astronomy (Iram), puts the challenge into perspective: “The signal in circular polarization is 100 times weaker than the unpolarized data we used to make the first black hole image. 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 objective was to test theoretical calculations and supercomputer simulations predicting the movements of plasmas in magnetic fields immersed in the gravitational field of a rotating black hole.
“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,” adds Abhishek Joshi, a graduate student at the University of Illinois.
“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,” concludes Angelo Ricarte, a postdoctoral researcher at CfA and member of the Harvard Black Hole Initiative.