The iconic image of a supermassive black hole in the Messier 87 (M87) galaxy—described by astronomers as a "fuzzy orange donut"—was a stunning testament to the capabilities of the Event Horizon Telescope (EHT). But there were still gaps in the observational data, limiting the resolution the EHT was able to achieve. Now four members of the EHT collaboration have applied a new machine-learning technique dubbed PRIMO (principal-component interferometric modeling) to the original 2017 data, giving that famous image its first makeover. They described their achievement in a new paper published in The Astrophysical Journal Letters.

“PRIMO is a new approach to the difficult task of constructing images from EHT observations,” said co-author Tod Lauer (NOIRLab). “It provides a way to compensate for the missing information about the object being observed, which is required to generate the image that would have been seen using a single gigantic radio telescope the size of the Earth.”

As we've reported previously, the EHT isn't a telescope in the traditional sense. Instead, it's a collection of telescopes scattered around the globe, including hardware from Hawaii to Europe, and from the South Pole to Greenland, though not all of these were active during the initial observations. The telescope is created by a process called interferometry, which uses light captured at different locations to build an image with a resolution that is the equivalent of a giant telescope (a telescope so big, it’s as if it were as large as the distance between the most distant locations of the individual telescopes).

Back in 2019, the EHT made headlines with its announcement of the first direct image of a black hole, located in the constellation of Virgo, some 55 million light years away. It was a feat that would have been impossible a mere generation ago, made possible by technological breakthroughs, innovative new algorithms, and of course, connecting several of the world's best radio observatories. Science magazine named the image its Breakthrough of the Year.

The EHT captured photons trapped in orbit around the black hole, swirling around at near the speed of light, creating a bright ring around it. From this, astronomers deduced that the black hole is spinning clockwise. The imaging also revealed the shadow of the black hole, a dark central region within the ring. That shadow is as close as astronomers can get to taking a picture of the actual black hole, from which light cannot escape once it crosses the event horizon. And just as the size of the event horizon is proportional to the black hole's mass, so, too, is the black hole's shadow: the more massive the black hole, the larger the shadow. It was a stunning confirmation of the general theory of relativity, showing that those predictions hold up even in extreme gravitational environments.

Two years later, the EHT released a new image of the same black hole, this time showing how it looked in polarized light. The ability to measure that polarization for the first time—a signature of magnetic fields at the black hole's edge—yielded fresh insight into how black holes gobble up matter and emit powerful jets from their cores. That polarization enabled astronomers to map the magnetic field lines at the inner edge and to study the interaction between matter flowing in and being blown outward.