by Astronomers have seen 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 could help scientists better understand the dark and silent cosmic titan around which our galaxy orbits.
A team of Michigan State University researchers made the groundbreaking discovery while combing through decades of NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) telescope. Nine large flares that the team discovered coming from Sgr A* were captured by NuSTAR, which has been observing the cosmos in X-rays since July 2012. The astronomers had previously been missing.
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“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 at Michigan State University’s Department of Physics and Astronomy, said in a statement. “Both flares and fireworks light up the dark and help us see 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 be 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 exit horizon. This represents the point at which the black hole’s gravitational pull becomes so strong 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. Thus, black holes are effectively invisible, only detectable by the effect they have on the matter around them – which can be catastrophic in the case of supermassive black holes.
Some of these cosmic titans are surrounded by massive amounts of general matter that they consume; others chew stars that pass too close to the event horizon. The massive gravitational pull of the black hole destroys those stars before dinner is made.
In both cases, however, material around the black hole eventually forms a flattened cloud, or “accretion disk,” with the black hole at its center. This disk glows intensely across the electromagnetic spectrum due to the turbulence and friction created by the intense tidal forces of the black hole.
However, not all the material in an accretion disk is fed to the supermassive central black hole. Some charged particles are sent to the poles of the black hole, 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 so bright they can outshine the combined light of all the stars in the galaxies around them.
Furthermore, not all supermassive black holes sit in AGNs and act as the central engines of quasars. Some are not surrounded by a wealth of gas, dust or unfortunate stars that get too close. This also means that they do not emit powerful bursts of light or have accretion discs, making them much harder to detect.
Sgr A*, which has a mass equal to about 4.5 million suns, happens to be one of these quiet, non-ravenous black holes. In fact, the cosmic titan at the heart of the Milky Way consumes as little matter as a person would eat just one grain of rice every million years or so.
When Sgr A* takes a bite, however, this is accompanied by a small X-ray flare. That’s exactly what the team set out to find 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 analysis, providing 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 this data bank on Sgr A* flares, we and other astronomers can analyze the properties of these X-ray flares and deduce the physical conditions inside the extreme environment of the supermassive black hole out,” Sanger-Johnson said.
Meanwhile, her colleague Jack Uteg, also from Michigan State University, was looking for something weaker and more subtle around Sarcel A*.
Black hole echoes around Sgr A*
Uteg examined the limited activity of Sarcel A* using a technique similar to listening to echoes. Looking at nearly 20 years of data, he pinpointed a massive molecular cloud near Sgr A* known as “the Bridge.”
Because clouds of gas and dust like these that flow between stars do not generate X-rays like the stars themselves, when astronomers detected these high-energy light emissions from the Bridge, they knew they must be coming from another source , and showing them there. from this molecular cloud.
“The brightness we see is probably the delayed reflection of X-ray bursts from Sgt A*,” explained Uteg. “We first noticed an increase in luminosity around 2008. Then, for the next 12 years, X-ray signals from the Bridge continued to increase until it reached peak brightness in 2020.”
It took hundreds of years for the light echo from the Bridge to travel to it from Sgr A* and then another 26,000 to travel to Earth. That means that by analyzing this X-ray echo, Uteg was able to begin to reconstruct 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 the Writer A* spawn has been in the past,” Uteg said. This indicated that about 200 years ago Sr A* was about 100,000 times brighter in X-rays than it is today.
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“This is the first time we have taken a 24-year variability for a molecular cloud around our supermassive black hole that has reached its peak X-ray brightness,” Zhang said. “It gives us the opportunity to tell the past activity of Sarcel A* around 200 years ago.
“Our research team at Michigan State University will continue this ‘game of astroarchaeology’ to further unravel the mysteries of the center of the Milky Way.”
One of the questions the team will be trying to answer is what is the exact mechanism that triggers the X-ray flares from Sarcel A*, given its short range. The researchers are confident that these findings will lead to further investigation by other teams, speculating that the findings could 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).
