By Ashley Strickland, CNN
There is a slumbering giant at the heart of our galaxy and occasionally, it wakes up and has an outburst.
© NASA, ESA, Gerald Cecil (UNC-Chapel Hill), Joseph DePasquale (STScI) This composite of view shows X-rays and warm, energetic gas near the center of the Milky Way galaxy. Glowing hydrogen gas is revealed in orange and streams show where a jet from the central black hole pushed through.
Astronomers have found evidence of this activity from the supermassive black hole at the heart of the Milky Way.
The black hole, which is 4 million times the mass of our sun, has the remains of a blowtorch-like jet of material from an outburst that occurred several thousand years ago.
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As black holes use their gravitational pull to tug material inward, interstellar gas and dust swirls into something called an accretion disk around the black hole. This swiftly rotating material heats up and blasts away from the black hole in jets that flare out across space at nearly the speed of light, accompanied by radiation.
Although our galaxy's black hole is often quiet, occasionally it releases activity, like cosmic burps and hiccups, as it gobbles up stars and gas clouds.
Astronomers used data from multiple telescopes to piece together this astronomical blast from the past to find that the jet's expelled material is still making its mark. A study detailing the findings published last week in the Astrophysical Journal.
In 2013, researchers detected X-rays and radio waves, using NASA's Chandra X-ray Observatory in space and the Jansky Very Large Array telescope in New Mexico, that suggested a jet was penetrating gas near the black hole.
This caused Gerald Cecil, a professor in the physics and astronomy department at the University of North Carolina in Chapel Hill, to question if there may be another jet radiating from the black hole in another direction.
Data taken from ground and space-based telescopes, including the Hubble Space Telescope, across multiple wavelengths of light essentially allowed Cecil to see an otherwise invisible and glowing hot bubble of gas that lined up about 35 light-years away from the black hole, as well as an expanding knot of gas that is only 15 light-years away.
When the jets strike gas clouds in the galaxy, the clouds react to the heat by expanding. Material within the gas clouds cause the jet to bend and split off into streams.
"The streams percolate out of the Milky Way's dense gas disk," said Alex Wagner, study coauthor and assistant professor at Tsukuba University in Japan, in a statement. "The jet diverges from a pencil beam into tendrils, like that of an octopus."
These streams led to a chain of expanding gas bubbles that extend for at least 500 light-years, a daisy chain that allowed the researchers to reconstruct past events.
"Like in archeology, you dig and dig to find older and older artifacts until you come upon remnants of a grand civilization," Cecil said.
When Wagner and Cecil ran computer models of the jets within the Milky Way, they were able to reproduce the data from the telescopes.
The black hole at the center of our galaxy is "currently powered down," Cecil said. But if it becomes active again, the jet will likely fire up again, too, and astronomers could observe how far the jet may reach, he said.
Astronomers have found evidence of this activity from the supermassive black hole at the heart of the Milky Way.
The black hole, which is 4 million times the mass of our sun, has the remains of a blowtorch-like jet of material from an outburst that occurred several thousand years ago.
Advertisement
As black holes use their gravitational pull to tug material inward, interstellar gas and dust swirls into something called an accretion disk around the black hole. This swiftly rotating material heats up and blasts away from the black hole in jets that flare out across space at nearly the speed of light, accompanied by radiation.
Although our galaxy's black hole is often quiet, occasionally it releases activity, like cosmic burps and hiccups, as it gobbles up stars and gas clouds.
Astronomers used data from multiple telescopes to piece together this astronomical blast from the past to find that the jet's expelled material is still making its mark. A study detailing the findings published last week in the Astrophysical Journal.
In 2013, researchers detected X-rays and radio waves, using NASA's Chandra X-ray Observatory in space and the Jansky Very Large Array telescope in New Mexico, that suggested a jet was penetrating gas near the black hole.
This caused Gerald Cecil, a professor in the physics and astronomy department at the University of North Carolina in Chapel Hill, to question if there may be another jet radiating from the black hole in another direction.
Data taken from ground and space-based telescopes, including the Hubble Space Telescope, across multiple wavelengths of light essentially allowed Cecil to see an otherwise invisible and glowing hot bubble of gas that lined up about 35 light-years away from the black hole, as well as an expanding knot of gas that is only 15 light-years away.
When the jets strike gas clouds in the galaxy, the clouds react to the heat by expanding. Material within the gas clouds cause the jet to bend and split off into streams.
"The streams percolate out of the Milky Way's dense gas disk," said Alex Wagner, study coauthor and assistant professor at Tsukuba University in Japan, in a statement. "The jet diverges from a pencil beam into tendrils, like that of an octopus."
These streams led to a chain of expanding gas bubbles that extend for at least 500 light-years, a daisy chain that allowed the researchers to reconstruct past events.
"Like in archeology, you dig and dig to find older and older artifacts until you come upon remnants of a grand civilization," Cecil said.
When Wagner and Cecil ran computer models of the jets within the Milky Way, they were able to reproduce the data from the telescopes.
The black hole at the center of our galaxy is "currently powered down," Cecil said. But if it becomes active again, the jet will likely fire up again, too, and astronomers could observe how far the jet may reach, he said.
We May Finally Know The Cause of 'The Cow', a Freakishly Exciting Space Explosion
The cause of a mysterious cosmic kaboom – so bright it led to the classification of a new type of space explosion – may have now been revealed.
The cause of a mysterious cosmic kaboom – so bright it led to the classification of a new type of space explosion – may have now been revealed.
© SDSS The location of AT2018cow, in a galaxy called CGCG 137-068.
According to an analysis of the 2018 event, nicknamed "the Cow" (AT2018cow), it was likely an unusual kind of core-collapse supernova that led to the formation of a compact cosmic object, either a neutron star or a small black hole.
"We have likely discovered the birth of a compact object in a supernova," says astronomer Dheeraj Pasham of MIT's Kavli Institute for Astrophysics and Space Research.
"This happens in normal supernovae, but we haven't seen it before because it's such a messy process. We think this new evidence opens possibilities for finding baby black holes or baby neutron stars."
The Cow was detected on 16 June 2018, and was immediately fascinating. It was incredibly brief, and incredibly bright, around 100 times brighter than a typical supernova. That's so bright that the Cow was initially thought to be coming from within the Milky Way. Astronomers were stunned when they figured out it actually emanated from a galaxy 200 million light-years away.
Since the Cow, more explosions with a similar profile have been identified. They have been named Fast Blue Optical Transients, of FBOTs, and astronomers have been keen to get to the bottom of what causes them.
One potential option was a tidal disruption flare from a black hole consuming another dense object, such as a white dwarf; or from an intermediate-mass black hole greater than 850 times the mass of the Sun stripping material from a passing star.
Another option was a type of core-collapse supernova, in which a stellar core, no longer supported by the outward pressure of fusion, collapses under its own gravity into an ultra-dense object.
One way to determine which of these scenarios was the most likely was to take a closer look at the X-ray data, so this is what Pasham and his team did.
"This signal was close and also bright in X-rays, which is what got my attention," Pasham says. "To me, the first thing that comes to mind is, some really energetic phenomenon is going on to generate X-rays. So, I wanted to test out the idea that there is a black hole or compact object at the core of the Cow."
The data they analyzed was from NASA's X-ray telescope Neutron Star Interior Composition Explorer (NICER), which is attached to the International Space Station. After the detection of the Cow, NICER observed the object for about 60 days to collect X-ray data on its post-nova behavior.
In those data, the researchers found that something within the Cow was pulsing in soft X-rays, letting out a burst every 4.4 milliseconds, for the entire duration of the 60-day observing period. This periodicity sets pretty stringent constraints on the physical mechanism producing the X-rays; whatever it is can be no larger than 1,000 kilometers (621 miles) across.
"The only thing that can be that small is a compact object – either a neutron star or black hole," Pasham says.
The strength of the signal also places constraints on the object's mass. It can be no greater than 800 times the mass of the Sun, which rules out tidal disruption of an intermediate-mass black hole. This also suggests a core-collapse.
The periodic pulsations could be produced by different mechanisms, depending on what the compact object is. If it's a neutron star, 4.4 milliseconds could be its spin rate. If it's a black hole, the emission could be produced by fallback – material blasted out during the supernova falling back into the newborn black hole, generating X-ray emissions.
There are still some unanswered questions that remain with either model, however. For a neutron star, the narrowness of the frequency range of the emissions is difficult to explain. For a black hole, characteristics such as the X-ray brightness and stability are difficult to explain.
Future studies of the Cow and other FBOTs could help to resolve these outstanding problems.
And they could also help us better understand some of the most extreme objects in the Universe.
"Whenever there's a new phenomenon, there's excitement that it could tell something new about the Universe," Pasham notes.
"For FBOTs, we have shown we can study their pulsations in detail, in a way that's not possible in the optical. So, this is a new way to understand these newborn compact objects."
The research has been published in Nature Astronomy.
According to an analysis of the 2018 event, nicknamed "the Cow" (AT2018cow), it was likely an unusual kind of core-collapse supernova that led to the formation of a compact cosmic object, either a neutron star or a small black hole.
"We have likely discovered the birth of a compact object in a supernova," says astronomer Dheeraj Pasham of MIT's Kavli Institute for Astrophysics and Space Research.
"This happens in normal supernovae, but we haven't seen it before because it's such a messy process. We think this new evidence opens possibilities for finding baby black holes or baby neutron stars."
The Cow was detected on 16 June 2018, and was immediately fascinating. It was incredibly brief, and incredibly bright, around 100 times brighter than a typical supernova. That's so bright that the Cow was initially thought to be coming from within the Milky Way. Astronomers were stunned when they figured out it actually emanated from a galaxy 200 million light-years away.
Since the Cow, more explosions with a similar profile have been identified. They have been named Fast Blue Optical Transients, of FBOTs, and astronomers have been keen to get to the bottom of what causes them.
One potential option was a tidal disruption flare from a black hole consuming another dense object, such as a white dwarf; or from an intermediate-mass black hole greater than 850 times the mass of the Sun stripping material from a passing star.
Another option was a type of core-collapse supernova, in which a stellar core, no longer supported by the outward pressure of fusion, collapses under its own gravity into an ultra-dense object.
One way to determine which of these scenarios was the most likely was to take a closer look at the X-ray data, so this is what Pasham and his team did.
"This signal was close and also bright in X-rays, which is what got my attention," Pasham says. "To me, the first thing that comes to mind is, some really energetic phenomenon is going on to generate X-rays. So, I wanted to test out the idea that there is a black hole or compact object at the core of the Cow."
The data they analyzed was from NASA's X-ray telescope Neutron Star Interior Composition Explorer (NICER), which is attached to the International Space Station. After the detection of the Cow, NICER observed the object for about 60 days to collect X-ray data on its post-nova behavior.
In those data, the researchers found that something within the Cow was pulsing in soft X-rays, letting out a burst every 4.4 milliseconds, for the entire duration of the 60-day observing period. This periodicity sets pretty stringent constraints on the physical mechanism producing the X-rays; whatever it is can be no larger than 1,000 kilometers (621 miles) across.
"The only thing that can be that small is a compact object – either a neutron star or black hole," Pasham says.
The strength of the signal also places constraints on the object's mass. It can be no greater than 800 times the mass of the Sun, which rules out tidal disruption of an intermediate-mass black hole. This also suggests a core-collapse.
The periodic pulsations could be produced by different mechanisms, depending on what the compact object is. If it's a neutron star, 4.4 milliseconds could be its spin rate. If it's a black hole, the emission could be produced by fallback – material blasted out during the supernova falling back into the newborn black hole, generating X-ray emissions.
There are still some unanswered questions that remain with either model, however. For a neutron star, the narrowness of the frequency range of the emissions is difficult to explain. For a black hole, characteristics such as the X-ray brightness and stability are difficult to explain.
Future studies of the Cow and other FBOTs could help to resolve these outstanding problems.
And they could also help us better understand some of the most extreme objects in the Universe.
"Whenever there's a new phenomenon, there's excitement that it could tell something new about the Universe," Pasham notes.
"For FBOTs, we have shown we can study their pulsations in detail, in a way that's not possible in the optical. So, this is a new way to understand these newborn compact objects."
The research has been published in Nature Astronomy.
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