Astronomers have detected 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, silent cosmic titan around which our galaxy orbits.
The team of researchers at Michigan State University made this groundbreaking discovery while analyzing decades of NASA data. Nuclear Spectroscopic Telescope Array (NuSTAR) telescope. Nine large flares that the team discovered coming from Mrs* They had been captured by NuSTAR, which has been observing the cosmos in X-rays since July 2012. These signals had not previously been detected by astronomers.
“We have a front-row seat to watching these unique cosmic fireworks at the center of our own galaxy, the Milky Way,” said team leader Sho Zhang, an assistant professor in the Department of Physics and Astronomy at Michigan State University. he said in a statement. “Both sparklers and fireworks illuminate the darkness and help us observe things we normally wouldn't be able to do.
“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 4th of July
Supermassive black holes as Mrs* They are believed to exist at the heart of all large galaxies. Like all black holes, supermassive black holes with masses equivalent to millions, or sometimes billions, of suns are surrounded by an outer boundary called event horizon. This marks the point at which the black hole's gravitational influence becomes so intense that not even light is fast enough to match its escape velocity.
This means that the event horizon acts as a unidirectional surface that traps light beyond which it is impossible to see. Therefore, black holes are effectively invisible and only detectable by the effect they have on the matter around them, which, in the case of supermassive black holes, can be catastrophic.
Some of these cosmic titans are surrounded by large amounts of general matter on which they feed; others chew stars that venture too close to the event horizon. Those stars are destroyed by the black hole's immense gravitational influence before becoming dinner.
In both cases, however, the matter eventually around the black hole forms a flattened cloud, or “accretion disk“, with the black hole at its center. This disk shines brightly across the electromagnetic spectrum due to the turbulence and friction created by the black hole's intense tidal forces.
Not all matter in an accretion disk is fed to the central supermassive black hole, however. Some charged particles are channeled toward the poles of the black hole, where they are ejected in the form of jets close to the speed of light that are also accompanied by bright electromagnetic radiation.
As a result, these voracious supermassive black holes are found in regions called active galactic nuclei (AGN)feeding quasars which are so bright that they can eclipse the combined light of all the stars in the surrounding galaxies.
Furthermore, not all supermassive black holes are found in AGN and act as central drivers of quasars. Some are not surrounded by a lot of gas, dust, or unfortunate stars that get too close. This also means they don't emit powerful bursts of light or have bright accretion disks, making them much harder to detect.
Sgr A*, with a mass equivalent to about 4.5 million suns, turns out to be one of these silent, non-voracious black holes. In fact, the cosmic titan at the heart of the Milky Way consumes so little matter that it is equivalent to a human eating just one grain of rice every million years or so.
However, when Sgr A* receives a small snack, this is accompanied by a faint flash of X-rays. That's exactly what the team set out to look for in 10 years of data collected by NuSTAR between 2015 and 2024.
Grace Sanger-Johnson of Michigan State University focused on spectacular bursts of high-energy light for the analysis, which provide a unique opportunity to study the immediate environment around the black hole. As a result, she found nine examples of these extreme outbreaks.
“We hope that by building this database on Sgr A* flares, we and other astronomers will be able to analyze the properties of these X-ray flares and infer the physical conditions within the extreme environment of the supermassive black hole,” Sanger-Johnson said. .
Meanwhile, his colleague Jack Uteg, also at Michigan State University, was looking for something weaker and more subtle around Sgr A*.
The black hole resonates around Sgr A*
Uteg examined the limited activity of Sgr A* using a technique similar to listening to echoes. After analyzing almost 20 years of data, he pointed to a giant molecular cloud near Sgr A* known as “the Bridge.”
Because clouds of gas and dust like this floating between stars don't generate X-rays like stars themselves do, when astronomers detected these high-energy light emissions from the Bridge, they knew they must be coming from another source and then be reflected. of this molecular cloud.
“The glow we see is probably the delayed reflection of previous X-ray bursts from Sgr A*,” Uteg explained. “We first observed an increase in luminosity around 2008. Then, over the next 12 years, the Bridge's X-ray signals continued to increase until reaching its maximum brightness in 2020.”
The light that resonates from the Bridge took hundreds of years to travel to it from Sgr A* and then took another 26,000 to travel to Earth. That means that by analyzing this X-ray echo, Uteg has been able to begin to piece together the recent cosmic history of our supermassive black hole.
“One of the main reasons we are concerned about this cloud getting brighter is that it allows us to limit the brightness of the Sgr A* burst in the past,” Uteg said. This revealed that about 200 years ago, Sgr A* was about 100,000 times brighter in X-rays than it is today.
“This is the first time we have constructed a 24-year variability for a molecular cloud surrounding our supermassive black hole that has reached its maximum X-ray luminosity,” Zhang said. “It allows us to recount the past activity of Sgr A* for about 200 years.
“Our research team at Michigan State University will continue this 'astroarchaeology game' to further unravel the mysteries of the center of the Milky Way.”
One of the puzzles the team will try to answer is what exact mechanism it is triggering. Sgr A* X-ray flares, given their poor diet. The researchers are confident that these findings will lead to further research by other teams, speculating that the results have the potential to 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).
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