By monitoring the high-energy X-ray emission from the center of our Galaxy, NuSTAR probes the current and past activity of Sagittarius A*, the supermassive at the center of the Milky Way. These images capture a flare observed in July 2012 over a 2-day period. In the middle panel, the black hole was consuming and heating matter to temperatures up to 180 million degrees Fahrenheit (100 million degrees Celsius). Image Credit: NASA/JPL-Caltech.
Black holes are notoriously difficult to study, in part because not even light can escape their immense gravity. Researchers typically infer their properties by observing the gravitational influence of a black hole on nearby stars, gas, energetic plasmas, and other related phenomena. Astronomers also learn both about the environment around the black hole and its past activity by observing echoes reflected off nearby structures (with the caveat that astronomers might have a different understanding of "nearby" than other people). Using X-ray data from NuSTAR, researchers at Michigan State University (MSU) have made groundbreaking discoveries about Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. These findings were presented at the 244th meeting of the American Astronomical Society, held in Madison, Wisconsin in June.
Galactic fireworks
Grace Sanger-Johnson, a postbaccalaureate researcher at MSU, analyzed 10 years of NuSTAR data looking for X-ray flares from Sagittarius A*. In doing so, she discovered nine flares that had previously gone unnoticed. These flares are dramatic bursts of high-energy light that provide a unique opportunity to study the environment around the black hole. This region is typically invisible due to the dense gas and dust that blocks most light, but not the penetrating, high-energy X-rays observed by NuSTAR. Sagittarius A* is the closest supermassive black hole to Earth. Data from Sagittarius A*, and its flares, are one of the only ways currently known to study the physics and physical environment of a black hole. Sanger-Johnson meticulously sifted through a decade's worth of X-ray data collected between 2015 and 2024 by NuSTAR. Each of the nine newly-discovered flares provides unique insight into the black hole's environment and activities.
"We are sitting in the front row to observe these unique cosmic fireworks at the center of our own Milky Way galaxy," said Prof. Shuo Zhang, Sanger-Johnson's advisor. "Flares light up the darkness and help us observe things we wouldn't normally be able to." Analyzing the properties of these X-ray flares will help astronomers infer the physical conditions of the extreme environment around the supermassive black hole.
"We are sitting in the front row to observe these unique cosmic fireworks at the center of our own Milky Way galaxy," said Prof. Shuo Zhang, Sanger-Johnson's advisor. "Flares light up the darkness and help us observe things we wouldn't normally be able to." Analyzing the properties of these X-ray flares will help astronomers infer the physical conditions of the extreme environment around the supermassive black hole.
`Echoes' of a black hole
While Sanger-Johnson focused on the brilliant flares from Sagittarius A*, Jack Uteg, an undergraduate researcher, examined the black hole's activity using a technique akin to listening to echoes. Uteg analyzed more than 20 years of data targeting a giant molecular cloud known as "the Bridge" near Sagittarius A*. This monitoring began before Uteg was born.
Unlike the central black hole, clouds of gas and dust in interstellar space cannot generate their own X-rays. So, when X-ray telescopes detected photons from the Bridge, the emission was inferred to be delayed reflection of past X-ray outbursts by Sagittarius A*. The X-ray luminosity of the Bridge first began to increase around 2008 and then increased over the next 12 years until it hit peak brightness in 2020. This "echo" light from the black hole traveled for hundreds of years from Sgr A* to the molecular cloud, and then traveled another roughly 26,000 years before reaching Earth.
By analyzing these X-ray echoes, Uteg was able to reconstruct a timeline of our black hole's past activity, offering insights that would not be possible through direct observations alone. Uteg's analysis used data from NuSTAR as well as from the European Space Agency's XMM-Newton satellite, which studies the universe in lower energy X-rays.
"One of the main reasons we care about this cloud getting brighter is that it lets us constrain how bright Sagittarius A* was in the past," Uteg said. Based on these data, the team determined that, about 200 years ago, the black hole at the center of the Milky Way was about 10,000 times brighter in X-rays compared to how we see it today. As the United States was celebrating its first July 4th, Sagittarius A* was feasting on nearby material, heating it up, and producing copious amounts of X-rays. Today, the black hole just nibbles. "The black hole at the center of our Galaxy was producing fireworks just 200 years ago, but today it is merely a sparkler," said Dr. Brian Grefenstette, a NuSTAR staff scientist at the California Institute of Technology.
This is the first long-term X-ray variability study of a molecular cloud surrounding Sagittarius A* to detect its peak X-ray luminosity. "This allows us to tell the past activity of Sgr A*, and we will continue this astro-archaeological study to further unravel the mysteries of the Milky Way's center," Zhang said.
While the exact mechanisms triggering X-ray flares and the life cycles of black holes remain mysteries and areas of active study, NuSTAR is helping spark further investigations and improve our understanding of these enigmatic objects.
Unlike the central black hole, clouds of gas and dust in interstellar space cannot generate their own X-rays. So, when X-ray telescopes detected photons from the Bridge, the emission was inferred to be delayed reflection of past X-ray outbursts by Sagittarius A*. The X-ray luminosity of the Bridge first began to increase around 2008 and then increased over the next 12 years until it hit peak brightness in 2020. This "echo" light from the black hole traveled for hundreds of years from Sgr A* to the molecular cloud, and then traveled another roughly 26,000 years before reaching Earth.
By analyzing these X-ray echoes, Uteg was able to reconstruct a timeline of our black hole's past activity, offering insights that would not be possible through direct observations alone. Uteg's analysis used data from NuSTAR as well as from the European Space Agency's XMM-Newton satellite, which studies the universe in lower energy X-rays.
"One of the main reasons we care about this cloud getting brighter is that it lets us constrain how bright Sagittarius A* was in the past," Uteg said. Based on these data, the team determined that, about 200 years ago, the black hole at the center of the Milky Way was about 10,000 times brighter in X-rays compared to how we see it today. As the United States was celebrating its first July 4th, Sagittarius A* was feasting on nearby material, heating it up, and producing copious amounts of X-rays. Today, the black hole just nibbles. "The black hole at the center of our Galaxy was producing fireworks just 200 years ago, but today it is merely a sparkler," said Dr. Brian Grefenstette, a NuSTAR staff scientist at the California Institute of Technology.
This is the first long-term X-ray variability study of a molecular cloud surrounding Sagittarius A* to detect its peak X-ray luminosity. "This allows us to tell the past activity of Sgr A*, and we will continue this astro-archaeological study to further unravel the mysteries of the Milky Way's center," Zhang said.
While the exact mechanisms triggering X-ray flares and the life cycles of black holes remain mysteries and areas of active study, NuSTAR is helping spark further investigations and improve our understanding of these enigmatic objects.
