Astronomers spotted flares 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 and silent cosmic Titan around which our galaxy orbits.
The team of researchers from Michigan State University made the breakthrough discovery while combing through decades of data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR). Nine large flares the team detected coming from Sgr A* were picked up 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 Shaw Zhang, an assistant professor in Michigan State University’s Department of Physics and Astronomy, said in a statement. “Both torches and fireworks illuminate the darkness and help us see things we normally wouldn’t 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 this light.”
Illumination of Sagittarius A* as Fourth of July
Supermassive black holes like Sgr A* are thought 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 influence becomes so intense that even light is not 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. Black holes are thus effectively invisible, detectable only through the effect they have on the matter around them – which in the case of supermassive black holes can be catastrophic.
Some of these space titans are surrounded by vast amounts of common matter that they feed on; others chew stars that dare to get too close to the event horizon. These stars are torn apart by the black hole’s massive gravitational pull before they become dinner.
In both cases, however, the eventual matter surrounding the black hole forms a flattened cloud or “accretion disk,” with the black hole sitting at its center. This disk glows brightly in the electromagnetic spectrum due to the turbulence and friction that the black hole’s intense tidal forces create.
However, not all the matter in the accretion disk is fed to the central supermassive black hole. Some charged particles are directed toward the poles of the black hole, where they are shot out as near-light-speed jets that are also accompanied by bright electromagnetic radiation.
As a result, these voracious supermassive black holes are found in regions called active galactic nuclei (AGN), powering quasars that are 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 AGN 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 bursts of light or have luminous accretion disks, making them much harder to detect.
Sgr A*, with a mass equivalent to about 4.5 million suns, turns out to be one of these quiet, non-predatory black holes. In fact, the space 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* gets a bit of a snack, it’s accompanied by a faint X-ray flare. That’s exactly what the team set out to look for in 10 years of data collected by NuSTAR from 2015 to 2024.
Grace Sanger-Johnson of Michigan State University focused on dramatic bursts of high-energy light for the analysis, which provides a unique opportunity to study the immediate environment around the black hole. As a result, she found nine examples of these extreme eruptions.
“We hope that by building this data bank 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,” Sanger-Johnson said. .
Meanwhile, her colleague Jack Uteg, also at Michigan State University, was looking for something fainter and subtler around Sgr A*.
Black hole echoes around Sgr A*
Uteg probes the limited activity of Sgr A* using a technique similar to listening for echoes. Looking at almost 20 years of data, he zeroed in on 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 the way the stars themselves do, when astronomers detected these high-energy light emissions from the Bridge, they realized they must be coming from another source, then reflect from this molecular cloud.
“The brightness we see is most likely the delayed reflection of past X-ray bursts from Sgr A*,” Uteg explained. “We first observed an increase in brightness around 2008. Then, over the next 12 years, the X-ray signals from the Bridge continued to increase until it reached a peak brightness in 2020.”
The light echoing off the Bridge took hundreds of years to reach it from Sgr A* and then another 26,000 years to reach 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 care about this cloud getting brighter is that it allows us to constrain how bright Sgr A*’s 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. “This 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 center of the Milky Way.”
One of the mysteries the team will seek to answer is what exactly is the mechanism that causes X-ray bursts from Sgr A*, given its meager diet. The researchers are confident that these findings will lead to further investigation 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 at the 244th meeting of the American Astronomical Society on Tuesday (June 11).
