Enlarge / This new, sharper image of the M87 supermassive black hole was generated by the PRIMO algorithm using 2017 EHT data. (credit: Medeiros et al. 2023)

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 similar to that of a telescope the size of the most distant locations.

Enlarge / Artist's impression of a supermassive black hole trailing stars behind it. (credit: NASA, ESA, Leah Hustak )

If you saw a similar streak in one of your photos, you'd probably take a few moments to clean off the lens. But the streak, in this case, was in an image taken by the Hubble Space Telescope, which is not affected by the schmutz that daily life leaves on Earth-bound hardware. So, a team of researchers decided to figure out what the long, thin smear might represent.

They're still not certain, but the best explanation appears to be the wake left behind by a supermassive black hole that's been shot free of the galaxy that used to host it. Its liberation likely resulted from two additional supermassive black holes, all brought together by a merger of galaxies. If this is right, it'll be the first instance of this behavior we've ever seen.

What is that?

Back in the days of film cameras, when it was sometimes possible to go months or even years between taking a photo and getting it developed, it wasn't unusual to pick up your newly developed snapshots and find yourself wondering what it was you had taken a picture of. You can almost hear echoes of those days in astronomers' description of seeing the smear across one of Hubble's images: "an almost-straight, thin streak was readily apparent in a visual assessment of the data quality."

Enlarge / The CHIME telescope has proven adept at picking up fast radio bursts. (credit: Andre Renard / CHIME Collaboration )

By combing through a collection of data, researchers may have discovered evidence that we've already observed the first "blitzar," a bizarre astronomical event caused by the sudden collapse of an overly massive neutron star. The event is driven by an earlier merger of two neutron stars; this creates an unstable intermediate neutron star, which is kept from collapsing immediately by its rapid spin. In a blitzar, the strong magnetic fields of the neutron star slow down its spin, causing it to collapse into a black hole several hours after the merger.

That collapse suddenly deletes the dynamo powering the magnetic fields, releasing their energy in the form of a fast radio burst. The researchers who performed the analysis suggest that this phenomenon could explain the non-repeating forms of these events.

Too big to live

How big can a neutron star get before it collapses into a black hole? We don't have a good answer, in part because we're not sure what happens to the bizarre forms of matter inside one of these massive objects. We don't even know if the neutrons that give the star its name survive or fall apart into their component quarks. It's one of those annoying questions where the answer includes the phrase "it depends."

Enlarge / Illustration of a star being spaghettified as it’s sucked in by a supermassive black hole during a tidal disruption event (TDE). (credit: ESO/M. Kornmesser)

Earlier this year, astronomers picked up an unusually bright signal in the X-ray, optical, and radio regimes, dubbed AT 2022cmc. They've now determined that the most likely source of that signal is a supermassive black hole gobbling up a star in a "hyper-feeding frenzy," shooting out jets of matter in what's known as a tidal disruption event (TDE). According to a new paper published in the journal Nature Astronomy, it's one for the record books: the furthest such event yet detected at roughly 8.5 billion light-years away.

The authors estimate the jet from this TDE is traveling at 99.99 percent the speed of light, meaning the black hole is really chowing down on its stellar repast. “It’s probably swallowing the star at the rate of half the mass of the sun per year,” said co-author Dheeraj “DJ” Pasham of the University of Birmingham. “A lot of this tidal disruption happens early on, and we were able to catch this event right at the beginning, within one week of the black hole starting to feed on the star.”

As we've reported previously , it's a popular misconception that black holes behave like cosmic vacuum cleaners , ravenously sucking up any matter in their surroundings. In reality, only stuff that passes beyond the event horizon—including light—is swallowed up and can't escape, although black holes are also messy eaters. That means that part of an object's matter is ejected in a powerful jet.

Enlarge / Simulation of two black holes poised on the verge of a collision. (credit: Simulating eXtreme Spacetimes (SXS) project )

The advent of gravitational wave detectors—there are now four of them—has produced a steady flow of black hole mergers. As far as we can tell, almost all of them have behaved exactly as we would expect for the sorts of events that we had predicted would produce them: a pair of orbiting black holes that gradually spiral inward until they meet at their mutual center of gravity.

But there was one event that apparently didn't quite match the sorts of signals we would expect. And researchers are now suggesting it was the product of something that should be incredibly rare: two black holes finding each other in the vastness of space. After a single close pass, the two bodies curved around and immediately swung into a collision.

Templates and chirps

Black hole collisions require that the two black holes be close enough to each other to gravitationally interact. Since space is so vast, this would typically mean that they are the products of two massive stars that formed as a binary system. After the stars died and left black holes behind, the two bodies would slowly spiral in toward each other, radiating away energy in the form of gravitational waves as they do.

Enlarge / To create the first image of the Milky Way’s black hole, scientists ran numerous simulations of the swirling envelope of plasma that encircles it. (credit: Knowable Magazine (CC-BY-ND) )

Black holes keep their secrets close. They imprison forever anything that enters. Light itself can’t escape a black hole’s hungry pull.

It would seem, then, that a black hole should be invisible —and taking its picture impossible. So great fanfare accompanied the release in 2019 of the first image of a black hole. Then, in spring 2022, astronomers unveiled another black hole photo—this time of the one at the center of our own Milky Way .

The image shows an orange, donut-shaped blob that looks remarkably similar to the earlier picture of the black hole in the center of galaxy Messier 87. But the Milky Way’s black hole, Sagittarius A*, is actually much smaller than the first and was more difficult to see, since it required peering through the hazy disk of our galaxy. So even though the observations of our own black hole were conducted at the same time as M87’s, it took three additional years to create the picture. Doing so required an international collaboration of hundreds of astronomers, engineers, and computer scientists and the development of sophisticated computer algorithms to piece together the image from the raw data.

On the morning of October 9, multiple space-based detectors picked up a powerful gamma-ray burst (GRB) passing through our solar system, sending astronomers around the world scrambling to train their telescopes on that part of the sky to collect vital data on the event and its afterglow. Dubbed GRB 221009A, astronomers say the gamma-ray burst is the most powerful yet recorded and likely could be the "birth cry" of a new black hole. The event was promptly published in the Astronomer's Telegram, and observations are still ongoing.

“In our research group, we’ve been referring to this burst as the ‘BOAT,’ or Brightest Of All Time, because when you look at the thousands of bursts gamma-ray telescopes have been detecting since the 1990s, this one stands apart,” said Jillian Rastinejad , a graduate student at Northwestern University. Rastinejad led one of two independent teams using the Gemini South telescope in Chile to study the event's afterglow.

“This burst is much closer than typical GRBs, which is exciting because it allows us to detect many details that otherwise would be too faint to see,” said Roberta Pillera , a graduate student at the Polytechnic University of Bari, Italy, and member of the Fermi Large Area Telescope (LAT) Collaboration. “But it’s also among the most energetic and luminous bursts ever seen regardless of distance, making it doubly exciting.”

Enlarge / Artist’s illustration of a tidal disruption event where a supermassive black hole spaghettifies and gobbles down a star. (credit: DESY, Science Communication Lab)

Back in October 2018, astronomers spotted the bright flare of a star being shredded by a black hole 20 million times more massive than our Sun 665 million light years away—a so-called " tidal disruption event " (TDE) dubbed AT2018hyz . But otherwise the event seemed unremarkable, and after a few months of monitoring the black hole in visible light, the TDE faded, and astronomers moved on. But AT2018hyz had a surprise in store. Nearly three years later, the black hole suddenly reanimated, baffling astronomers, according to a new paper published in The Astrophysical Journal.

“This caught us completely by surprise—no one has ever seen anything like this before,” said co-author Yvette Cendes of the Harvard-Smithsonian Center for Astrophysics. She likened the unusual black hole feeding behavior to “burping” after a heavy meal. "It's as if this black hole has started abruptly burping out a bunch of material from the star it ate years ago." This suggests that delayed outflow is more common than astronomers previously expected. The group will continue to monitor this TDE as it evolves, and a systematic study of a much larger sample of TDEs is underway.

As we've reported previously , it's a popular misconception that black holes behave like cosmic vacuum cleaners , ravenously sucking up any matter in their surroundings. In reality, only stuff that passes beyond the event horizon—including light—is swallowed up and can't escape, although black holes are also messy eaters. That means that part of an object's matter is actually ejected out in a powerful jet.

Enlarge / If a star (red trail) wanders too close to a black hole (left), it can be shredded, or spaghettified, by the intense gravity. Some of the star’s matter swirls around the black hole, like water down a drain, emitting copious X-rays (blue). (credit: NASA/CXC/M. Weiss)

When astronomers first observed a star that was shredded, or “spaghetified,” after approaching too close to a massive black hole in 2019, they determined that much of the star’s matter was launched outward in a powerful wind from the optical light emitted from the blast. Now, astronomers from the University of California, Berkeley (UCB) have analyzed the polarization of that light to determine that the cloud was likely spherically symmetric, adding further evidence for the presence of that powerful wind.

“This is the first time anyone has deduced the shape of the gas cloud around a tidally spaghetiffied star,” said co-author Alex Filippenko , a UCB astronomer. The latest findings appeared in a recent paper published in the Monthly Notices of the Royal Astronomical Society.

As we've reported previously , an object that passes beyond the event horizon of a black hole—including light—is swallowed up and can't escape, although black holes are also messy eaters. That means that part of an object's matter is actually ejected out in a powerful jet. If that object is a star, the process of being shredded (or "spaghetified") by the powerful gravitational forces of a black hole occurs outside the event horizon, and part of the star's original mass is ejected violently outward. This can form a rotating ring of matter (aka an accretion disk ) around the black hole that emits powerful X-rays and visible light. The jets are one way astronomers can indirectly infer the presence of a black hole.