In April of 2019, the Event Horizon Telescope (EHT) collaboration 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. This year 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 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, the researchers have 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.
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.
Source: “How Researchers Produce Sharp Images of a Black Hole” Universe Today, 24 March 2020.<https://www.universetoday.com/145443/how-researchers-produce-sharp-images-of-a-black-hole/>