In October 2019, a high-energy neutrino crashed into Antarctica.
The neutrino, which is very difficult to detect, caught the attention of astronomers: What could produce such a powerful particle?
The researchers traced the neutrino to a supermassive black hole that had just ruptured and swallowed a star. Known as the tidal disruption event (TDE), AT2019dsg occurred in the same region of the sky where the neutrino came from – in April 2019 – just months ago. The horribly violent event must have been the source of the powerful particle, the astronomers said.
But new research casts doubt on this claim.
In a study published this month in the Astrophysical Journal, researchers from the Center for Astrophysics | Harvard & Smithsonian and Northwestern University provide extensive new radio observations and data on AT2019dsg, allowing the team to calculate the energy emitted by the event. The findings show that AT2019dsg is produced nowhere near the energy required for the neutrino; In fact, what it came up with was pretty “ordinary,” the team concludes.
Black Holes Are Scatter Eaters
As counterintuitive as it may seem, black holes don’t always swallow everything within reach.
“Black holes are not like vacuum cleaners,” says Yvette Cendes, a postdoctoral researcher at the Center for Astrophysics, who led the research.
When a star orbits too close to a black hole, gravitational forces begin to stretch or spaghetti the star, Cendes explains. Eventually, the elongated material orbits the black hole and heats up, creating a glow in the sky that astronomers can see from millions of light-years away.
“But when there’s too much material, black holes can’t eat it all at once,” says Kate Alexander, a co-author and postdoctoral researcher at Northwestern University who calls black holes “scatter eaters.” “During this process, some of the gas is repulsed – for example, when babies are eating, some of the food gets on the floor or walls.”
These residues are ejected back into space in the form of an outlet or jet – which, if powerful enough, could theoretically produce a subatomic particle known as a neutrino.
A Possible Source for Neutrinos
Using the Very Large Array in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the team was able to observe AT2019dsg about 750 million light-years away for more than 500 days after the black hole began to consume the star. . Extensive radio observations make the AT2019dsg the best-studied TDE to date, and revealed that radio luminosity peaked about 200 days after the event began.
According to the data, the total amount of energy in the outflow was equivalent to the energy emitted by the Sun for 30 million years. While this may sound impressive, the powerful neutrino detected on October 1, 2019 would require a source 1,000 times more energetic.
“Instead of seeing the bright material jet required for this, we’re seeing a weaker material ejection from the radio,” explains Alexander. “Instead of a strong fire hose, we see a gentle wind.”
“If this neutrino somehow came from AT2019dsg, it begs the question: Why didn’t we detect neutrinos associated with supernovae at this distance or closer? They are much more common and have the same energy velocities,” Cendes said.
The team concludes that the neutrino is unlikely to come from this particular TDE. However, astronomers are far from understanding how they initiate TDEs and neutrinos.
“We’ll probably check this again,” says Cendes, who believes there is still much to learn. “This particular black hole is still feeding.”
TDE AT2019dsg was first discovered on April 9, 2019, by the Zwicky Temporary Facility in Southern California. The neutrino, known as IceCube-191001A, was detected six months later by the IceCube Neutrino Observatory at the South Pole.
Additional study co-authors Edo Berger and Peter Williams of the Center for Astrophysics; Tarraneh Eftekhari of Northwestern University; and Ryan Chornock of the University of California, Berkeley.
About the Center for Astrophysics | Harvard and Smithsonian
Center for Astrophysics | Harvard & Smithsonian is a collaboration between Harvard and Smithsonian designed to ask and ultimately answer humanity’s biggest unsolved questions about the nature of the universe. The Center for Astrophysics is headquartered in Cambridge, MA, and has research facilities in the US and around the world.
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