Black Hole Blowback
There’s a black hole pointed right at us — and apparently, that’s a good thing.
December 19, 2022 | Written by Jackie Appel
In February of this year, researchers spotted the appearance of one of the brightest lights in the sky. Luckily, because they were already observing the sky in the direction of the event, they were able to gather substantial data about the flare.
Several months later, that data has been processed, and astronomers have announced what they think they saw. According to a recent study led by researchers from multiple institutions, the flash was caused by a rarely-spotted phenomenon called a jetted tidal disruption event pointed directly at Earth.
“This is the fourth known — and so far best-studied — example of a jet pointed at us in such an event,” said Tanmoy Laskar, an astronomer at Harvard University and one of the authors on the study.
Jetted tidal disruption events are rarely spotted for a few reasons. To see one at all, you first have to have a tidal disruption event, which is what happens when a massive star gets too close to a supermassive black hole. The gravity of the black hole begins to rip the star apart, causing the star to “stretch out” in a process called spaghettification. Researchers estimate this happens about once every 10,000 years per galaxy.
Most of the time, these events result in about half the star being added to a black hole’s accretion disk —the ring of debris around a black hole that makes it look like a glowing donut — and half being pulled past the event horizon and into the gravity well. But sometimes, the part of the star that doesn’t get pulled into the black hole gets spat out at nearly the speed of light.
These events produce what we call relativistic jets, and the appearance of them turns a tidal disruption event into a jetted tidal disruption event. This isn’t the only way that jets can form. For instance, they can be produced by the gamma-ray bursts that sometimes come from dying or merging stars, or from blazars — the results of supermassive black holes consuming a bunch of the gas in their galaxies.
But jetted tidal disruption events are their own beast. Which brings us to the second reason they are so rarely spotted: they have to be pointed right at us in order for us to see them.
“We have discovered about one hundred tidal disruption events, and only four had a jet pointed towards Earth,” said Matteo Lucchini, an astronomer at MIT and another author on the paper. “Which makes sense – ‘pointing towards Earth’ is a much less likely direction than ‘pointing anywhere else in the sky but towards Earth.’”
“If the jet were perpendicular, we likely wouldn't even know that the jet was there,” said Laskar. “We wouldn't see any light from it, because of the Doppler effect.”
The Doppler effect comes into play quite a bit in astronomy, and it can either really help or really hurt a researcher’s attempts at observation. It’s the phenomenon that makes sound coming toward you sound higher-pitched than sound moving away from you — like you might notice happening when an ambulance drives by.
And it doesn’t only work on sound. It affects all waves, including light. Basically, light moving towards us gets squished and compounded so that by the time it reaches us, it registers as extremely bright. Light moving in any other direction doesn’t benefit from that amplification, so it’s a lot harder to see, and we’re more likely to just miss it altogether.
But in the case of observing this event, we got lucky. We not only had the Doppler effect on our side — we had a sky survey pointed in the right direction.
“There is a difference between ‘how many sources do we observe?’ versus ‘how many are out there?’” said Lucchini. “This is one of the reasons this particular event was so exciting - it is the first to be discovered by a new sky survey called ZTF (Zwicky Transient Facility), which promises to be much more efficient at discovering exotic transients than what we have had available in the past.”
Spotting as many of these events as we can is good news for astronomers for one major reason in particular: we don’t know how jets like this are formed in tidal disruption events. There are certainly theories — one of the leading ones being that the gravity of the black hole affects the magnetic fields of objects being destroyed so much that the fields get twisted and shoot matter out and away from the black hole — but we aren’t entirely sure.
“Finding the face-on jet tells us something about how many jets we are likely missing because they are not pointed at us, and that helps us figure out how many of these events produce jets, which, in turn, can help us understand what causes the jets,” said Laskar.
And finding out what causes unexplained phenomena is the name of the game in astronomy. So, researchers will be keeping their eyes out in years to come. As sky surveys get better and better at detecting these events, researchers will get closer and closer to understanding this dramatic and mysterious phenomenon.
Meet the Writer
Jackie Appel is a freelance writer for Continuum with a degree in astrophysics from Ohio State University and a master’s degree in Science, Health and Environmental Reporting from New York University. She has previously written for Scienceline and loves sharing the weird wonders of the universe with anyone who wants to listen. Jackie thinks “spaghettification” is the best science word ever (and a very cool phenomenon!).
Have long thought that the accretion disk of a black hole is capable of pulling so much mass in from all directions that a portion must escape along its rotational axes driven to extreme velocities by the incoming compression close to the Schwarzschild radius. (there isn't enough kerning in this font to separate the c from the l or the r from the n, somebody should fix that)