‘Flares’ shed light on the Milky Way’s supermassive black hole

By using data from NASA’s NuSTAR X-ray telescope, a team of researchers from Michigan State University (MSU) has recently made groundbreaking discoveries about the supermassive black hole at the center of our Milky Way galaxy. 

Black holes, known for their immense gravity that even light cannot escape, are typically studied by observing their effects on nearby stars and gas clouds. 

Dynamic environment at the heart of our galaxy

Grace Sanger-Johnson and Jack Uteg, led by Shuo Zhang, an assistant professor in the Department of Physics and Astronomy, have innovatively used decades of X-ray data from space-based telescopes to further understand these cosmic entities. 

“Grace and Jack’s contributions are a source of immense pride,” Zhang said. “Their work exemplifies MSU’s commitment to pioneering research and fostering the next generation of astronomers.” 

“This research is a prime example of how MSU scientists are unlocking the universe’s secrets, bringing us closer to comprehending the nature of black holes and the dynamic environment at the heart of our galaxy.”

Flares from the Milky Way’s black hole

Sanger-Johnson analyzed ten years of data looking for X-ray flares from Sagittarius A* (Sgr A*), the Milky Way’s central black hole, discovering nine previously unnoticed flares. These high-energy flares offer a unique opportunity to study the black hole’s immediate environment, a region usually invisible due to its gravity. 

“We are sitting in the front row to observe these unique cosmic fireworks at the center of our own Milky Way galaxy. Both flares and fireworks light up the darkness and help us observe things we wouldn’t normally be able to,” noted Zhang.

“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.”

Gaining insights from the x-ray flares

Sanger-Johnson meticulously reviewed a decade’s worth of X-ray data collected from 2015 to 2024 by NuSTAR. Each of the nine newly discovered flares provides invaluable data for understanding the black hole’s environment and activities. 

“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,” she said.

Echoes of ancient light

While Sanger-Johnson focused on the flares, Uteg used a different approach, examining the black hole’s activity by analyzing nearly 20 years of data targeting a giant molecular cloud known as “the bridge” near Sgr A*. 

“Unlike stars, these clouds of gas and dust in interstellar space don’t generate their own X-rays,” explained Uteg. “The brightness we see is most likely the delayed reflection of past X-ray outbursts from Sgr A*.” 

“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.”

This “echo” light from the black hole, traveling for hundreds of years from Sgr A* to the molecular cloud and another 26,000 years to Earth, allows scientists to reconstruct a timeline of the black hole’s past activity. 

Historical brightness of the black hole

Uteg’s analysis, using data from NuSTAR and the European Space Agency’s X-ray Multi-Mirror (XMM) Newton space observatory, helps understand the black hole’s historical brightness. 

“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,” said Uteg.

Unraveling mysteries at the center of the Milky Way 

From their calculations, the team determined that about 200 years ago, Sgr A* was approximately five orders of magnitude 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. It allows us to tell the past activity of Sgr A* from about 200 years ago,” said Zhang.

“Our research team at MSU will continue this ‘astroarchaeology game’ to further unravel the mysteries of the Milky Way’s center.”

While the exact mechanisms triggering X-ray flares and the precise life cycle of black holes remain mysterious, the experts are confident their findings will spark further investigation and potentially revolutionize our understanding of these enigmatic objects.

The findings were presented at the 244th meeting of the American Astronomical Society (AAS) on June 11. 

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