Something is glowing in Sagittarius A* (Sgr A*), the supermassive black hole in the heart of our Milky Way galaxy. In a multi-institutional study, astronomers have created the first three-dimensional visualization of a high-energy flare from Sgr A*. This achievement, accomplished using data collected by the Atacama Large Millimeter/Submillimeter Array (ALMA), provides unique insights into the turbulent surroundings of black holes.
“This is the first three-dimensional reconstruction of gas rotating close to a black hole,” says Katie Bouman, assistant professor of computing and mathematical sciences, electrical engineering and astronomy at Caltech, whose group led the effort described in a new paper in Nature Astronomy.
Black holes and flares
On April 11, 2017, ALMA observed mysterious flares coming from the area near Sgr A*. These flares, which appear irregularly and change in strength, are thought to come from the black hole’s accretion disk—where gravity pulls in dust and gas, which then spiral inwards. While these flares are usually observed in various wavelengths like X-ray, infrared, and radio, making a three-dimensional display of their shape has been difficult until now.
The newest research studied the data using orbital polarimetric tomography, a method that combines neural network-based 3D representations with intricate gravitational models. This allows scientists to see these flares in three dimensions for the first time.
ALMA does not only capture a single light curve. This is because the telescope records data on different light polarization states, resulting in several “videos” for each observation. Polarization, similar to wavelength and intensity, is a fundamental characteristic of light that shows how the electric part of a light wave aligns with the wave’s direction of travel.
“What we get from ALMA is two polarized single-pixel videos,” Bouman said. “That polarized light is actually really, really informative.”
Aviad Levis, a postdoctoral scholar in Bouman’s group and the primary author of the paper, emphasizes that the video is not a simulation nor a direct recording of real-time events.
“It is a reconstruction based on our models of black hole physics. There is still a lot of uncertainty associated with it because it relies on these models being accurate,” Levis said.
The 3D reconstruction showed two bright, compact areas circling around six times the radius from the event horizon of Sgr A*. The visualization suggests these areas move in a clockwise direction in a disk with a low tilt relative to our line of sight. This matches the predictions from previous theoretical studies and helps confirm some of the existing models about black hole behavior.
“This is very exciting,” Bouman said. “It didn’t have to come out this way. There could have been arbitrary brightness scattered throughout the volume. The fact that this looks a lot like the flares that computer simulations of black holes predict is very exciting.”
Creating a 3D model of these flares was difficult because of the strong gravitational forces that distort space-time and, in turn, affect the path of light. The light had to travel through highly warped space-time, adding layers of complexity to the analysis.
To address this, the team developed computational imaging tools that could handle the bending of light due to gravitational effects—a phenomenon known as gravitational lensing. This process was necessary for accurately interpreting the limited two-dimensional data collected from ALMA.
This discovery has major implications for astrophysics, offering a new way to understand the dynamics of accretion disks near supermassive black holes. These detailed insights into the behavior of flares can help improve our understanding and refinement of black hole accretion physics models. The technique could also potentially be used to study other occurrences in the universe, like the surroundings of other black holes and neutron stars.
“This is a really interesting application of how AI and physics can come together to reveal something that is otherwise unseen,” Levis said. “We hope that astronomers could use it on other rich time-series data to shed light on complex dynamics of other such events and to draw new conclusions.”
