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Discovery of Hotspots Circling Milky Way Black Hole Has Astronomers Excited

The discovery of  wobbling "hotspots" circling the drain of a massive black hole offers exciting new evidence for the behemoth that lies at our galaxy's center — and the astronomer who first worked on this theory shares how 13 years of observations have finally paid off.

The new study, involving the work of by Avery Broderick, an astronomer from the University of Waterloo in Ontario, Canada, revealed three flares, or visual hotspots, emanating from the Milky Way's central black hole, also known as Sagittarius A*

The team detected a wobble of emissions coming from the flares, allowing the scientists to detect the accretion disk — a growing mass of orbiting gas and debris — surrounding the black hole itself. In turn, the researchers were able to use the emissions to map the behavior of Sagittarius A*, Broderick told [Images: Black Holes of the Universe]

Broderick's black hole theory built on earlier research by two teams that studied the galactic center of the Milky Way in near-infrared. This included the work of Reinhard Genzel, an astronomer from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, as well as researchers Andrea Ghez and Mark Morris of University of California, Los Angeles. At the time, their work revealed that the center of the Milky Way wasn't steady, but instead would drastically brighten about once a day for about 30 or 40 minutes, Broderick said. 

Researchers think supermassive black holes exist at the center of most, if not all, large galaxies. Therefore, in 2005, while working alongside researcher Avi Loeb at the Harvard-Smithsonian Center for Astrophysics, Broderick argued that a periodic brightening observed at the heart of the Milky Way, also known as a bright infrared flare, was the result of an incredibly massive object such as a black hole.

This theory was further supported by evidence of a very bright, dense group of stars called a nuclear star cluster that surrounds the central region of the Milky Way. In addition, infrared observations showed that the stars at the very center of the galaxy orbit a dark object estimated to be 4 million solar masses in size, again suggesting the presence of a black hole, Broderick said. 

Even still, there was not enough data to prove that a black hole truly exists at the center of the Milky Way, Broderick added — until now. 

Sudden flare behavior 

The three flares recently detected emanate from the central region of the Milky Way and are the product of gravitational lensing by the black hole, Broderick said.

"The black hole acts like the lens of a lighthouse. [There is] a localized emission region, but it's not the sudden brightening of the emission region itself that causes the flare," Broderick said. Rather, the gravity of the black hole bends the light from the emission and magnifies it for us to see. "That's what's responsible for the flaring — the manifestation of extreme gravity." 

Based on this, Broderick and Loeb originally predicted that objects embedded in the accretion disk surrounding Sagittarius A* would exhibit a distinct wobble in their infrared emissions due to their orbital motion around the black hole. The researchers published their theory in a 2005 paper, as well as a 2006 follow-up study. However, at the time, the technology was not yet available to detect such a wobble, Broderick said. 

That all changed with the launch of the GRAVITY instrument on the European Southern Observatory's Very Large Telescope in 2016. The precision and sensitivity of the GRAVITY instrument helped astronomers detect a wobble in emissions coming from the three flares in the accretion disk circling Sagittarius A*. Their findings were published Oct. 18 in the journal Astronomy & Astrophysics. 

"While flares have been seen for a long time now … the key finding here is the characteristic wobble of the flares," which indicates that the material producing these flares is moving around a black hole, Broderick said.

The flares observed near Sagittarius A* occur when magnetic field lines close to the black hole break apart and reconnect. This process, also known as magnetic reconnection, releases large amounts of energy and charged particles, causing the telltale shine. The infrared emissions from the flares exhibit a characteristic wobble due to their orbital motion around the black hole. Specifically, as other emissions embedded in the accretion flow move around the galactic center, the center of light shifts, or "wobbles," Broderick said. 

The material circling just outside the black hole's event horizon whirls at roughly a third of the speed of light. The orbital period of the flare — the time it takes to complete one orbit — is the same as that of the wobble, which astronomers observed once every 40 to 50 minutes. The short time scale is a result of the strength of gravity from the black hole and suggests that the material is orbiting incredibly close to the black hole, Broderick said. 

"Thirteen years ago, our statement was that these flares were associated with the structural variability [of the galactic center] and that we were going to be able to use that structural variability to say something about general relativity and strong gravity," Broderick said. "The exciting thing is, that [statement] appears to be true." 

Proving there's a black hole 

The wobble of emissions coming from the flares is not the only sign suggesting that a supermassive black hole lies at the heart of the Milky Way. There's no doubt that a supermassive object with a mass about 4 million times that of the sun is located at our galactic core, but proving that it's a black hole has been challenging. 

However, the center of the galaxy is also home to a nuclear star cluster comprising more than 500,000 stars. Based on Albert Einstein's theory of general relativity, astronomers are able to estimate the mass of the object lurking at the center of the Milky Way by measuring the speed of the orbiting stars and gas. This further supports the idea that the 4-million-solar-mass object is a supermassive black hole, Broderick said. 

"There is no other object that we know of that could sustain all of that mass in such a compact configuration and not collapse," Broderick said. "If not a black hole, then it would have to be something extraordinarily exotic and outside the realm of what we understand today." 

Furthermore, observations have revealed that material really does disappear from our galactic center. As this material is drawn inward toward that center, it gets trapped in the black hole’s growing accretion disk. While most matter in the accretion disk orbits safely around Sagittarius A*, material that gets too close risks being drawn past the event horizon, the point beyond which it can't escape the black hole's gravitational pull. Researchers can see friction created by the material flowing toward the event horizon causing that material to brighten, Broderick said. 

"It becomes this sort of 'where did it go?' argument," Broderick said. "We have this material that we know is falling in towards a black hole, because we see its accretion luminosity, but we don't see the corresponding impact luminosity. Therefore, it must be disappearing somewhere."

The presence of an event horizon beyond which lies a black hole is therefore the most logical explanation for the behavior observed at our galactic center, Broderick said.

"It's extremely exciting on a personal level to see a prediction actually pan out," Broderick said. "I think this marks a moment in the history of science, where black holes are going from something of curiosity to something that is fundamentally real."

Editor's note: This story has been corrected from an earlier version that referenced Avery Broderick as the lead author of the new study. The new study was led by astronomers with the GRAVITY Collaboration and further supports the theory originally developed by Broderick in 2005.

