How Researchers Produce Sharp Images of a Black Hole

In April of 2019, the Event Horizon Telescope collaboration history made history when it released the first image of a black hole ever taken. This accomplishment was decades in the making and triggered an international media circus. The picture was the result of a technique known as interferometry, where observatories across the world combined light from their telescopes to create a composite image.

This image showed what astrophysicists have predicted for a long time, that extreme gravitational bending causes photons to fall in around the event horizon, contributing to the bright rings that surround them. Last week, on March 18th, a team of researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) announced new research that shows how black hole images could reveal an intricate substructure within them.

The study that describes their findings, titled “Universal interferometric signatures of a black hole’s photon ring,” recently appeared in the journal Science Advances. The team was led by Michael Johnson, an astrophysicist with the CfA, and induced members from Harvard’s Black Hole Initiative (BHI), Los Alamos National Laboratory, Princeton Center for Theoretical Science, and multiple universities.

As Johnson explained in a recent CfA press release:

“The image of a black hole actually contains a nested series of rings. Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we’ve caught just a glimpse of the full complexity that should emerge in the image of any black hole.”

As the law of General Relativity tells us, gravitational fields alter the curvature of spacetime. In the case of a black hole, the effect is extreme and causes even light (photons) to infall around them. These photons cast a shadow on the bright ring of infalling gas and dust that is accelerated to relativistic speeds by the black hole’s gravity.

Around this shadowed region is a “photon ring” produced from photons that are concentrated by the strong gravity near the black hole. This ring can tell astronomers a lot about a black hole’s since its size and shape reveal the mass and rotation (aka. “spin”) of the black hole. Because of the EHT images, black hole researchers now have a tool with which to study black holes.

Annotated image showing the interior structure of a black hole. Credit: George Wong (UIUC) and Michael Johnson (CfA)

Since the 1950s, astronomers have learned a great deal about them by studying the effect they have on their surrounding environment. In other words, the study of black holes has been indirect and theoretical in nature. But with the ability to take images of these celestial objects, astronomers can finally study them directly and glean real data.

George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign, was responsible for developing software to produce simulated black hole images. This software is what allowed for images that were of the highest resolution to date to be computed and allowed their team to decompose them into the predicted series of sub-images. As Wong indicated:

“Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation. What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits.”

What was especially surprising to the researchers, however, was how the substructure revealed by the black hole image creates new opportunities for research. While the subrings they revealed are normally invisible to the naked eye on images, they produce very clear signals when observed by arrays of telescopes using interferometry.

NASA visualization of a black hole (annotated). Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman

This presents astronomers with a relatively easy way to expand on the work conducted by the EHT collaboration thus far. “While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart,” said Johnson. “Adding one space telescope to the EHT would be enough.”

The fields of astronomy and astrophysics have experienced multiple revolutions in recent years. Between the first-ever observations of interstellar objects, the confirmation of gravitational waves, and the first direct observations of a black hole. These firsts have enabled research that promises to unlock a number of enduring mysteries about the cosmos.

The team’s research was made possible in part by grants issued by NASA, the National Science Foundation (NSF), the Department of Energy (DoE), and multiple scientific and research foundations.

Further Reading: CfA