Astronomers have spotted flares and echos 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 quiet cosmic titan around which our galaxy orbits.
The team of Michigan State University researchers made the groundbreaking discovery while combing through decades of data from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) telescope. Nine large flares the team discovered coming from Sgr A* had been caught by NuSTAR, which has been observing the cosmos in X-rays since July 2012. These signals had previously been missed by astronomers.
"We are sitting in the front row 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 flares and fireworks light up the darkness and help us observe things we wouldn't normally be able to.
"That's why astronomers need to know when and where these flares occur, so they can study the black hole's environment using that light."
Lighting up Sagittarius A* like the fourth of July
Supermassive black holes like 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 an event horizon. This marks the point at which the black hole's gravitational influence becomes so intense not even light is fast enough to match its escape velocity.
This means the event horizon acts as a one-way light-trapping surface beyond which it is impossible to see. Thus, black holes are effectively invisible, only detectable 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 vast amounts of general matter they feed upon; others chew on stars that venture too close to the event horizon. Those stars get shredded by the immense gravitational influence of the black hole before becoming dinner.
In both cases, however, 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 because of the turbulence and friction that the black hole's intense tidal forces create.
Not all of the matter in an accretion disk is fed to the central supermassive black hole, however. Some charged particles are channeled to the black hole's poles, where they are blasted out as near-light-speed jets that are also accompanied by bright electromagnetic radiation.
As a result, these ravenous supermassive black holes sit 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.
Furthermore, not all supermassive black holes sit in AGNs and act as the central engines of quasars. Some aren't surrounded by a wealth of gas, dust or unfortunate stars that get too close. This also means they don't emit powerful bursts of light or have glowing accretion disks, making them much trickier to detect.
Sgr A*, with a mass equivalent to around 4.5 million suns, just happens to be one of these quiet, non-ravenous black holes. In fact, the cosmic titan at the heart of the Milky Way consumes so little matter it is equivalent to a human eating just one grain of rice every million years or so.
When Sgr A* does get a little snack, however, this is accompanied by a faint X-ray flare. That's exactly what the team set about searching for in 10 years of data collected by NuSTAR from 2015 to 2024.
Michigan State University's Grace Sanger-Johnson focused on dramatic bursts of high-energy 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 flares.
"We hope that by building up this bank of data on Sgr A* flares, we and other astronomers can analyze the properties of these X-ray flares and infer the physical conditions inside the extreme environment of the supermassive black hole," Sanger-Johnson said.
Meanwhile, her colleague Jack Uteg, also from Michigan State University, was looking for something fainter and more subtle around Sgr A*.
Black hole echoes around Sgr A*
Uteg examined the limited activity of Sgr A* using a technique akin 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 drift between stars don't generate X-rays like stars themselves do, when astronomers detected these high-energy light emissions from the Bridge, they knew they must be coming from another source, then being reflected off this molecular cloud.
"The brightness we see is most likely the delayed reflection of past X-ray outbursts from Sgr A*," Uteg explained. "We first observed an increase in luminosity around 2008. Then, for the next 12 years, X-ray signals from the Bridge continued to increase until it hit peak brightness in 2020."
The light echoing from the Bridge took hundreds of years to travel to it from Sgr A* and then took another 26,000 to travel to Earth. That means 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 care about this cloud getting brighter is that it lets us constrain how bright the Sgr A* outburst was in the past," Uteg said. This revealed that around 200 years ago, Sgr A* was around 100,000 times brighter in X-rays than it is today.
"This is the first time that we have constructed a 24-year-long variability for a molecular cloud surrounding our supermassive black hole that has reached its peak X-ray luminosity," Zhang said. "It allows us to tell 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 unravel the mysteries of the Milky Way's center."
One of the riddles the team will seek to answer is what the exact mechanism is triggering X-ray flares from Sgr A*, given its sparse diet. The researchers are confident these findings will lead to further investigation by other teams, speculating that the results have the potential to revolutionize our understanding of the supermassive black holes and their environments.
The team presented their findings at the 244th meeting of the American Astronomical Society on Tuesday (June 11).
