Astronomers have detected 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 could help scientists better understand the dark and silent cosmic titan around which our galaxy orbits.
A team of Michigan State University researchers made a groundbreaking discovery while sifting through decades of data from NASA’s Nuclear Spectroscopic Telescope Array (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 were previously 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,” said team leader Sho Zhang, an assistant professor in Michigan State University’s Department of Physics and Astronomy. “Both flares and fireworks light up the darkness and help us see things we wouldn’t normally be able to.
“Therefore, astronomers need to know when and where these eruptions occur in order to study the black hole’s environment using this light.”
Sagittarius A* lighting up like the 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 pull is 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 that cannot be seen beyond. Black holes are therefore 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 space titans are surrounded by vast amounts of mundane matter that they feed on; others chew on stars that venture too close to the event horizon. These stars are destroyed by the massive gravitational pull of the black hole before they become dinner.
In both cases, however, the eventual mass around the black hole forms a flattened cloud, or “accretion disk”, in the center of which the black hole sits. This disk glows intensely across the electromagnetic spectrum due to the turbulence and friction that creates the black hole’s intense tidal forces.
However, not all the matter in the accretion disk is fed into the central supermassive black hole. Some charged particles are directed towards the poles of the black hole, from where they are shot out as jets with the speed of close to light, which are also accompanied by bright electromagnetic radiation.
As a result, these predatory supermassive black holes lie in regions called active galactic nuclei (AGN) and power 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 sit in AGNs and act as the central engines of quasars. Some aren’t surrounded by lots of gas, dust, or unlucky stars that get too close. This also means that they do not emit strong flashes of light or have glowing accretion disks, making them much more difficult to detect.
Sgr A*, with a mass equivalent to about 4.5 million suns, happens to be one of these silent, uneaten black holes. The space titan at the heart of the Milky Way actually consumes so little matter that it’s equivalent to a human eating just one grain of rice in a million years.
However, when Sgr A* gets a small snack, it is accompanied by a faint X-ray burst. That’s exactly what the team set out to find in 10 years of data collected by NuSTAR between 2015 and 2024.
Grace Sanger-Johnson of Michigan State University focused on the dramatic high-energy flashes of light for analysis, which provide a unique opportunity to study the immediate environment around a black hole. As a result, she found nine examples of these extreme flare-ups.
“We hope that by creating 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 a supermassive black hole,” Sanger-Johnson said.
Meanwhile, her colleague Jack Uteg, also of Michigan State University, was looking for something fainter and more subtle around Sgr A*.
Black hole reverberates around Sgr A*
Uteg investigated the limited activity of Sgr A* using a technique similar to listening to echoes. Looking at nearly 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 move between stars don’t produce X-rays like the stars themselves, when astronomers detected these high-energy light emissions from Bridge, they knew they must be coming from another source and then being reflected. from this molecular cloud.
“The brightness we see is most likely a delayed reflection of past X-ray bursts from Sgr A*,” explained Uteg. “We first observed an increase in luminosity around 2008. Then, for the next 12 years, the X-ray signals from the Bridge continued to grow until they reached their maximum brightness in 2020.”
Light bouncing off 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 was able to begin reconstructing the recent cosmic history of our supermassive black hole.
“One of the main reasons we care about making this cloud 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-long variability for the 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* 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 puzzles the team will try to answer is what exact mechanism triggers Sgr A*’s X-ray bursts, given its sparse diet. The researchers are confident that the findings will lead to further investigation by other teams and speculate that the results have the potential to change 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).