In October 2019, a high-energy neutrino slammed into Antarctica. The neutrino, which was remarkably hard to detect, piqued astronomers' interest: what could generate such a powerful particle? Researchers traced the neutrino back to a supermassive black hole that had just ripped apart and swallowed a star. Known as a tidal disruption event (TDE), AT2019dsg occurred just months earlier in April 2019 in the same region of the sky where the neutrino had come from. The monstrously violent event must have been the source of the powerful particle. But new research casts doubt on that claim.
In a study published this month in the Astrophysical Journal, researchers at the Center for Astrophysics | Harvard & Smithsonian and Northwestern University, present extensive new radio observations and data on AT2019dsg, allowing the team to calculate the energy emitted by the event. The findings show AT2019dsg generated nowhere near the energy needed for the neutrino; in fact, what it spewed out was quite ordinary. While it may seem counterintuitive, black holes do not always swallow everything in reach.
Yvette Cendes, a postdoctoral fellow at the Center for Astrophysics who led the study said, black holes are not like vacuum cleaners.
When a star wanders too close to a black hole, gravitational forces begin to stretch, or spaghettify, the star, Cendes explains. Eventually, the elongated material spirals around the black hole and heats up, creating a flash in the sky that astronomers can spot from millions of light-years away.
Kate Alexander, a study co-author and postdoctoral fellow at Northwestern University who calls black holes ']messy eaters. But when there's too much material, black holes can't eat it all smoothly at once. Some of the gas gets spewed back out during this process like when babies eat, some of the food ends up on the floor or the walls.
These leftovers get flung back into space in the form of an outflow, or jet which, if powerful enough, could theoretically generate a subatomic particle known as a neutrino. Using the Very Large Array in New Mexico and Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the team was able to observe AT2019dsg, some 750 million light-years away, for more than 500 days after the black hole had started consuming the star. The extensive radio observations make AT2019dsg the most well-studied TDE to date and revealed that the radio brightness peaked around 200 days after the event began.
According to the data, the total amount of energy in the outflow was equivalent to the energy radiated by the Sun over the course of 30 million years. While that may sound impressive, the powerful neutrino spotted on Oct. 1, 2019, would require a source 1,000 times more energetic.
Alexander said, instead of seeing the bright jet of material needed for this, we see a fainter radio outflow of material. Instead of a powerful firehose, we see a soft wind.
Cendes said, if this neutrino somehow came from AT2019dsg, it begs the question: Why haven't we spotted neutrinos associated with supernovae at this distance or closer? They are much more common and have the same energy velocities.
The team concludes it's unlikely that the neutrino came from this particular TDE. If it did, however, astronomers are far from understanding TDEs and how they launch neutrinos.
Cendes, who believes there's still much to learn and said, we're probably going to check in on this one again. This particular black hole is still feeding.
TDE AT2019dsg was first discovered on April 9, 2019, by the Zwicky Transient Facility in Southern California. The neutrino, known as IceCube-191001A, was detected by the IceCube Neutrino Observatory in the South Pole six months later.
Journal Information: Y. Cendes et al, Radio Observations of an Ordinary Outflow from the Tidal Disruption Event AT2019dsg, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/ac110a