Astronomers Discover 18 New Tidal Disruption Events

A tidal disruption event occurs when a star winds up so close to a supermassive black hole that the tidal forces exceed the star’s self-gravity and shred the star; as the black hole feasts, it gives off an enormous burst of energy across the electromagnetic spectrum.

“The majority of the new sources don’t show up in optical bands,” said Megan Masterson, a graduate student in MIT’s Kavli Institute for Astrophysics and Space Research.

“If you want to understand tidal disruption events (TDEs) as a whole and use them to probe supermassive black hole demographics, you need to look in the infrared band.”

Masterson and her colleagues recently detected the closest TDE yet, by searching through infrared observations.

The discovery opened a new, infrared-based route by which astronomers can search for actively feeding black holes.

That first detection spurred the group to comb for more TDEs.

For their new study, the researchers searched through archival observations taken by NASA’s NEOWISE mission.

They looked through the mission’s archived observations using an algorithm that picks out patterns in infrared emissions — likely signs of a transient burst of infrared radiation.

They then cross-referenced the flagged transients with a catalog of all known nearby galaxies within 600 million light-years.

They found that infrared transients could be traced to about 1,000 galaxies.

The authors then zoomed in on the signal of each galaxy’s infrared burst to determine whether the signal arose from a source other than a TDE, such as an active galactic nucleus or a supernova.

After ruling out these possibilities, the team then analyzed the remaining signals, looking for an infrared pattern that is characteristic of a TDE — namely, a sharp spike followed by a gradual dip, reflecting a process by which a black hole, in ripping apart a star, suddenly heats up the surrounding dust to about 1,000 K before gradually cooling down.

The team’s analysis revealed 18 clean signals of tidal disruption events.

The authors took a survey of the galaxies in which each TDE was found, and saw that they occurred in a range of systems, including dusty galaxies, across the entire sky.

“If you looked up in the sky and saw a bunch of galaxies, the TDEs would occur representatively in all of them,” Masteron said.

“It’s not that they’re only occurring in one type of galaxy, as people thought based only on optical and X-ray searches.”

“It is now possible to peer through the dust and complete the census of nearby TDEs,” said Harvard University’s Professor Edo Berger.

“A particularly exciting aspect of this work is the potential of follow-up studies with large infrared surveys, and I’m excited to see what discoveries they will yield.”

The new discoveries help to resolve some major questions in the study of TDEs.

For instance, prior to this work, astronomers had mostly seen TDEs in one type of galaxy — a post-starburst system that had previously been a star-forming factory, but has since settled.

This galaxy type is rare, and astronomers were puzzled as to why TDEs seemed to be popping up only in these rarer systems.

It so happens that these systems are also relatively devoid of dust, making a TDE’s optical or X-ray emissions naturally easier to detect.

Now, by looking in the infrared band, astronomers are able to see TDEs in many more galaxies.

The team’s new results show that black holes can devour stars in a range of galaxies, not only post-starburst systems.

The findings also resolve a missing energy problem. Physicists have theoretically predicted that TDEs should radiate more energy than what has been actually observed.

But the authors now say that dust may explain the discrepancy. They found that if a TDE occurs in a dusty galaxy, the dust itself could absorb not only optical and X-ray emissions but also extreme ultraviolet radiation, in an amount equivalent to the presumed ‘missing energy.’

The 18 new detections also are helping astronomers estimate the rate at which TDEs occur in a given galaxy.

When they figure the new TDEs in with previous detections, they estimate a galaxy experiences a tidal disruption event once every 50,000 years.

This rate comes closer to physicists’ theoretical predictions. With more infrared observations, the team hopes to resolve the rate of TDEs, and the properties of the black holes that power them.

“People were coming up with very exotic solutions to these puzzles, and now we’ve come to the point where we can resolve all of them,” said MIT’s Dr. Erin Kara.

“This gives us confidence that we don’t need all this exotic physics to explain what we’re seeing.”

“And we have a better handle on the mechanics behind how a star gets ripped apart and gobbled up by a black hole. We’re understanding these systems better.”

The team’s paper was published in the Astrophysical Journal.

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Megan Masterson et al. 2024. A New Population of Mid-infrared-selected Tidal Disruption Events: Implications for Tidal Disruption Event Rates and Host Galaxy Properties. ApJ 961, 211; doi: 10.3847/1538-4357/ad18bb