Astronomers have spotted bursts 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 could help scientists better understand the dark and silent cosmic titan that our Milky Way orbits.
The team of researchers from Michigan State University made the groundbreaking discovery while sifting through decades of data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) telescope. Nine large solar flares the team discovered, originating from Sgr A*, were captured by NuSTAR, which has been observing the cosmos in X-rays since July 2012. These signals had previously been missed by astronomers.
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“We have a front-row seat to observe these unique cosmic fireworks at the center of our own Milky Way Galaxy,” team leader Sho Zhang, an assistant professor at Michigan State University’s department of physics and astronomy, said in a statement. “Both torches and fireworks light up the darkness and help us observe things we wouldn’t normally be able to see.
“That’s why astronomers need to know when and where these outbursts occur so they can study the black hole’s environment with that light.”
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Supermassive black holes like Sgr A* are believed to exist at the core of all large galaxies. Like all black holes, supermassive black holes with masses equal 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 influence 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-way light-trapping surface beyond which it is impossible to see. So black holes are effectively invisible and only observable due to 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 enormous amounts of general matter on which they feed; others chew on stars that move too close to the event horizon. Those stars are torn apart by the black hole’s immense gravitational influence before they become dinner.
In both cases, however, the final matter surrounding the black hole forms an oblate cloud, or ‘accretion disk’, with the black hole at the center. This disk glows intensely across the entire electromagnetic spectrum due to the turbulence and friction created by the black hole’s intense tidal forces.
However, not all matter in an accretion disk is carried to the central supermassive black hole. Some charged particles are channeled to the black hole’s poles, where they are blown away as near-light-speed jets that are also accompanied by bright electromagnetic radiation.
As a result, these voracious supermassive black holes reside in regions called active galactic nuclei (AGN), feeding quasars so bright that they can outshine the combined light of every star in the galaxies around them.
Furthermore, not all supermassive black holes reside in AGNs and act as the central engines of quasars. Some aren’t surrounded by an abundance of gas, dust, or unlucky stars that get too close. This also means that they do not emit powerful flashes of light or have glowing accretion disks, making them much harder to detect.
Sgr A*, with a mass equivalent to about 4.5 million suns, happens to be one of these silent, non-voracious 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.
However, when Sgr A* is given a small snack, it is accompanied by a faint X-ray flare. That’s exactly what the team looked for in the ten years of data NuSTAR collected between 2015 and 2024.
Grace Sanger-Johnson of Michigan State University focused the analysis on dramatic bursts of high-energy light, which provide a unique opportunity to study the immediate environment around the black hole. As a result, she found nine examples of these extreme outbursts.
“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 in the extreme environment of the supermassive black hole,” says Sanger-Johnson. said.
Meanwhile, her colleague Jack Uteg, also from Michigan State University, was looking for something weaker and more subtle around Sgr A*.
Black hole echoes around Sgr A*
Uteg examined Sgr A*’s limited activity using a technique similar to listening to echoes. Looking at nearly two decades of data, he focused on a giant molecular cloud near Sgr A*, known as “the Bridge.”
Because clouds of gas and dust like these floating between stars don’t generate X-rays the way stars themselves do, when astronomers spotted these high-energy light emissions from the bridge they must have come from some other source and were then reflected. of this molecular cloud.
“The brightness we see is most likely the delayed reflection of past X-ray bursts from Sgr A*,” Uteg explains. “We first observed an increase in brightness around 2008. Over the next twelve years, the bridge’s X-ray signals continued to increase until reaching maximum brightness in 2020.”
It took hundreds of years for the light reflecting off the bridge to travel from Sgr A* to the bridge, 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 to reconstruct the recent cosmic history of our supermassive black hole.
“One of the main reasons we think it’s important for this cloud to brighten is so that it allows us to limit how bright the Sgr A* burst was in the past,” Uteg said. This showed that Sgr A* was about 100,000 times brighter on X-rays about 200 years ago than it is today.
“This is the first time we have constructed a 24-year variability for a molecular cloud around our supermassive black hole that has reached its maximum X-ray brightness,” said Zhang. “It allows us to recount the past activities of Sgr A* from about 200 years ago.
“Our research team at Michigan State University will continue this ‘astroarchaeological game’ to further unravel the mysteries of the center of the Milky Way.”
One of the riddles the team will try to answer is what the exact mechanism is that causes X-ray flares in Sgr A*, given its sparse diet. The researchers are confident that these findings will lead to further research by other teams, speculating that the results have the potential to revolutionize our understanding of supermassive black holes and their environments.
The team presented their findings Tuesday (June 11) at the 244th meeting of the American Astronomical Society.