Scientists investigated the past activity of Sagittarius A*, the supermassive black hole in our galaxy, by analyzing light echoes from X-ray bursts, which indicate frequent consumption of material in the past and a historically brighter state.

The supermassive black hole at the center of our Milky Way Galaxy is a silent beast. However, Sagittarius A* (or Sgr A* for short) is not completely inert. Every now and then it absorbs a bunch of molecular gas, or even a star, and then suffers from an upset stomach. This emits X-ray bursts into the surrounding area.

By sifting through decades of X-ray data, Michigan State University researcher Grace Sanger-Johnson found nine previously undiscovered X-ray bursts from Sagittarius A*, the central supermassive black hole of the Milky Way. This NASA image, released more than a decade ago, shows an example of an X-ray burst. Credit: NASA/JPL-Caltech

Grace Sanger-Johnson and Jack Uteg, two researchers from Michigan State University, studied flares and light echoes in detail. What they found shows the activity of Sgr A* in the very distant past, when Sgr A* was gobbling up matter. It took hundreds of years for the X-rays from this activity to pass through Sgr A*, reflect and illuminate a nearby molecular cloud. This created an echo of light that traveled for about another 26,000 years before reaching Earth. So when Uteg and Sanger examined these flares and light echoes, they were literally looking into the past.

Searching for Sgr A* X-ray bursts with NuSTAR

Sanger-Johnson analyzed decades of data, looking for X-ray flashes from Sgr A*’s eating habits. During the search, he found evidence of nine more such outbreaks.

Outbreaks are often quite dramatic. Because they are so bright, they allow astronomers to study the immediate environment around the black hole. The data Sanger-Johnson examined came from the NuSTAR mission. It focuses on high-energy X-ray and gamma radiation. They often come from active regions at the hearts of galaxies, supernova explosions and other active events.

The data Sanger-Johnson collected and analyzed is now a database of Sgr flares A. “By creating this database of Sgr A bursts, we hope that we and other astronomers will be able to analyze the properties of these X-ray flares and make inferences about the physical conditions in the extreme environment of a supermassive black hole,” Sanger-Johnson said. aforementioned.

Flash echo tracking

While Sanger-Johnson worked with the NuSTAR data, research student Jack Uteg examined activity around the black hole. He analyzed 20 years of data from a giant molecular cloud called a bridge. The data comes from observations made by NuSTAR and the European Space Agency’s XMM-Newton observatory. The bridge is located near Sgr A* and does not normally emit its own light.

According to Uteg, who created a timeline of Sgr’s past outbursts, astronomers noticed it glowing in X-rays. A. “The brightness we see is likely a delayed reflection of past X-ray bursts from Sgr A,” he said. “We first observed an increase in brightness around 2008. Then, over the next 12 years, X-ray signals from the bridge continued to increase until they reached a peak in 2020.”

Uteg’s work helped astronomers determine that Sgr A* was about five times brighter in X-rays than it is now. This illumination indicates that the supermassive black hole at our center is likely destroying a nearby gas cloud. Additionally, according to Uteg, brightness revealed other characteristics. “One of the main reasons we care about this cloud getting brighter is that it allows us to constrain how bright the Sgr A* flare has been in the past,” he said.

This light echoes from the Sgr A* show

Thanks to the work of Sanger-Brown and Uteg, astronomers have found another way to overcome the difficulties of observing around black holes. “Both torches and fireworks light up the darkness and help us see things we wouldn’t normally be able to see,” he said. “Therefore, astronomers need to know when and where these flashes occur so that they can study the surroundings of the black hole with this light.”

Astronomers know that a black hole gobbles up material around it on a variable basis, but these findings help them constrain how often this happens and how the resulting explosions affect the surrounding space. There are still many questions about how often these outbreaks occur and whether they have occurred in the past, according to MSU Associate Professor Shuo Zhang, who led the team on the two studies.

“This is the first time we have plotted the 24-year variability of the molecular cloud surrounding our supermassive black hole as it reaches its peak X-ray luminosity,” Zhang said. “This allows us to tell about the past activities of Sgr A*, about 200 years ago. Our research group at Moscow State University will continue this ‘astroarchaeological game’ to further unravel the mysteries of the center of the Milky Way.”