Astronomers have spotted flashes and echoes coming from the supermassive black hole at the heart of the Milky Way, Sagittarius A* (Sgr A*). These “cosmic fireworks” and X-ray echoes can help scientists better understand the dark, quiet cosmic titan around which our galaxy orbits.
The Michigan State University team of researchers made the groundbreaking discovery by digging through decades of data from NASA’s NuSTAR telescope (NuSTAR). The nine large flares the team discovered coming from Sgr A* were picked up by NuSTAR, which has been observing the cosmos in X-rays since July 2012. These signals had not previously been picked up by astronomers.
Connected: New view of supermassive black hole at heart of Milky Way hints at exciting hidden feature (image)
“We have a front-row seat to observe these unique cosmic fireworks at the center of our Milky Way galaxy,” team leader Sho Zhang, an assistant professor in Michigan State University’s Department of Physics and Astronomy, said in a statement. “Both flames and fireworks illuminate the darkness and help us observe things we would not normally be able to.
“That’s why astronomers need to know when and where these explosions occur, so they can study the black hole’s environment using that light.”
Sagittarius A* illumination as on the Fourth of July
Supermassive black holes such as Sgr A* are believed to exist at the hearts of all large galaxies. Like all black holes, supermassive black holes with masses equivalent to millions, or sometimes billions, of suns are surrounded by an outer boundary called the event horizon. This marks the point at which the black hole’s gravitational pull becomes so intense that not even light is fast enough to match its escape velocity.
This means that the event horizon acts as a one-sided light-catching surface beyond which it is impossible to see. Thus, black holes are effectively invisible, detectable only by the effect they have on the matter around them – which, in the case of supermassive black holes, can be catastrophic.
Some of these cosmic titans are surrounded by large quantities of the general matter on which they feed; others chew on stars that hover too close to the event horizon. Those stars are torn apart by the black hole’s immense gravitational pull before they become dinner.
However, in both cases, the eventual matter around the black hole forms a flattened cloud, or “accretion disk”, with the black hole sitting at its center. This disk glows intensely across the electromagnetic spectrum due to the turbulence and friction created by the black hole’s strong tidal forces.
However, not all the matter in an accretion disk feeds into the central supermassive black hole. Some charged particles are channeled to the poles of the black hole, where they explode as near-light-speed jets that are also accompanied by bright electromagnetic radiation.
As a result, these predatory supermassive black holes reside in regions called active galactic nuclei (AGN), powering quasars that are so bright they can outshine the combined light of every star in the galaxies around them.
Moreover, not all supermassive black holes reside in AGN and act as the central engines of quasars. Some are not surrounded by a large amount of gas, dust, or ill-fated stars that come too close. This also means they don’t emit powerful bursts of light or have glowing accretion discs, making them much trickier to detect.
Sgr A*, with a mass equivalent to about 4.5 million suns, just happens to be one of these quiet, non-creepy black holes. In fact, the cosmic titan at the heart of the Milky Way consumes so little matter that it is equivalent to a human eating just one grain of rice every million years or so.
However, when Sgr A* gets a snack, it’s accompanied by a faint burst of X-rays. That’s exactly what the team tried to look for in the 10 years of data collected by NuSTAR from 2015 to 2024.
Grace Sanger-Johnson of Michigan State University focused on the dramatic high-energy bursts of light for the analysis, which provide a unique opportunity to study the immediate environment around the black hole. As a result, she found nine examples of these extreme explosions.
“We hope that by building this database of Sgr A* flares, we and other astronomers can analyze the properties of these X-ray flares and infer the physical conditions within the extreme environment of the supermassive black hole,” Sanger-Johnson . said.
Meanwhile, her colleague Jack Uteg, also of Michigan State University, was looking for something fainter and more subtle about Sgr A*.
Black hole echoes around Sgr A*
Uteg examined the limited activity of Sgr A* using a technique similar to listening to echoes. Looking at almost 20 years of data, he targeted a giant molecular cloud near Sgr A* known as “The Bridge”.
Because clouds of gas and dust like this that move between stars don’t generate X-rays the way the stars themselves do, when astronomers detected these high-energy light emissions from Ura, they knew they had to come from a different, more powerful source. after they reflected. from this molecular cloud.
“The glow we see is most likely the delayed reflection of past X-ray bursts from Sgr A*,” explained Uteg. “We first noticed an increase in brightness around 2008. Then, for the next 12 years, the X-ray signals from the Bridge continued to increase until it reached its peak brightness in 2020.”
Light echoing from the Bridge took hundreds of years to travel to it from Sgr A*, and then another 26,000 years to travel to Earth. This means that by analyzing this X-ray echo, Uteg has been able to begin reconstructing the recent cosmic history of our supermassive black hole.
“One of the main reasons we’re interested in this cloud getting brighter is that it allows us to constrain how bright the Sgr A* outburst was in the past,” Uteg said. This revealed that about 200 years ago, Sgr A* was about 100,000 times brighter in X-rays than it is today.
“This is the first time we have constructed a 24-year variability for a molecular cloud surrounding our supermassive black hole that has reached its peak X-ray luminosity,” Zhang said. “It allows us to show the past activity of Sgr A* from about 200 years ago.
“Our research team at Michigan State University will continue this ‘astroarchaeology game’ to further uncover the mysteries of the Milky Way’s center.”
One of the puzzles the team will seek to answer is what exact mechanism is causing the X-ray flares from Sgr A*, given its rarefied diet. The researchers are confident that these findings will lead to further investigations by other teams, speculating that the results have the potential to revolutionize our understanding of supermassive black holes and their environments.
The team presented its findings at the 244th meeting of the American Astronomical Society on Tuesday (June 11).