WORD OF THE DAY; SPAGHETTIFICATION
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A star’s unexpected survival
A team of physicists devise a model that maps a star’s surprising orbit about a supermassive black hole – revealing new information about one of the cosmos’ most extreme environments.
Peer-Reviewed PublicationIMAGE: THIS ILLUSTRATION SHOWS A GLOWING STREAM OF MATERIAL FROM A STAR AS IT IS BEING DEVOURED BY A SUPERMASSIVE BLACK HOLE IN A TIDAL DISRUPTION FLARE. WHEN A STAR PASSES WITHIN A CERTAIN DISTANCE OF A BLACK HOLE - CLOSE ENOUGH TO BE GRAVITATIONALLY DISRUPTED - THE STELLAR MATERIAL GETS STRETCHED AND COMPRESSED AS IT FALLS INTO THE BLACK HOLE. view more
CREDIT: NASAJPL-CALTECH
Hundreds of millions of light-years away in a distant galaxy, a star orbiting a supermassive black hole is being violently ripped apart under the black hole’s immense gravitational pull. As the star is shredded, its remnants are transformed into a stream of debris that rains back down onto the black hole to form a very hot, very bright disk of material swirling around the black hole, called an accretion disc. This phenomenon – where a star is destroyed by a supermassive black hole and fuels a luminous accretion flare – is known as a tidal disruption event (TDE), and it is predicted that TDEs occur roughly once every 10,000 to 100,000 years in a given galaxy.
With luminosities exceeding entire galaxies (i.e., billions of times brighter than our Sun) for brief periods of time (months to years), accretion events enable astrophysicists to study supermassive black holes (SMBHs) from cosmological distances, providing a window into the central regions of otherwise-quiescent - or dormant - galaxies. By probing these ``strong-gravity’’ events, where Einstein's general theory of relativity is critical for determining how matter behaves, TDEs yield information about one of the most extreme environments in the universe: the event horizon – the point of no return – of a black hole.
TDEs are usually “once-and-done” because the extreme gravitational field of the SMBH destroys the star, meaning that the SMBH fades back into darkness following the accretion flare. In some instances, however, the high-density core of the star can survive the gravitational interaction with the SMBH, allowing it to orbit the black hole more than once. Researchers call this a repeating partial TDE.
A team of physicists, including lead author Thomas Wevers, Fellow of the European Southern Observatory, and co-authors Eric Coughlin, assistant professor of physics at Syracuse University, and Dheeraj R. “DJ” Pasham, research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, have proposed a model for a repeating partial TDE. Their findings, published in Astrophysical Journal Letters, describe the capture of the star by a SMBH, the stripping of the material each time the star comes close to the black hole, and the delay between when the material is stripped and when it feeds the black hole again. The team’s work is the first to develop and use a detailed model of a repeating partial TDE to explain the observations, make predictions about the orbital properties of a star in a distant galaxy, and understand the partial tidal disruption process.
The team is studying a TDE known as AT2018fyk (AT stands for ``Astrophysical Transient’’). The star was captured by a SMBH through an exchange process known as “Hills capture,” where the star was originally part of a binary system (two stars that orbit one another under their mutual gravitational attraction) that was ripped apart by the gravitational field of the black hole. The other (non-captured) star was ejected from the center of the galaxy at speeds comparable to ~ 1000 km/s, which is known as a hypervelocity star.
Once bound to the SMBH, the star powering the emission from AT2018fyk has been repeatedly stripped of its outer envelope each time it passes through its point of closest approach with the black hole. The stripped outer layers of the star form the bright accretion disk, which researchers can study using X-Ray and Ultraviolet /Optical telescopes that observe light from distant galaxies.
According to Wevers, having the opportunity to study a partial TDE gives unprecedented insight into the existence of supermassive black holes and the orbital dynamics of stars in the centers of galaxies.
“Until now, the assumption has been that when we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be fatal for the star, that is, the star is completely destroyed,” he says. “But contrary to all other TDEs we know of, when we pointed our telescopes to the same location again several years later, we found that it had re-brightened again. This led us to propose that rather than being fatal, part of the star survived the initial encounter and returned to the same location to be stripped of material once more, explaining the re-brightening phase.”
First detected in 2018, AT2018fyk was initially perceived as an ordinary TDE. For approximately 600 days the source stayed bright in the X-ray, but then abruptly went dark and was undetectable - a result of the stellar remnant core returning to a black hole, explains MIT physicist Dheeraj R. Pasham.
“When the core returns to the black hole it essentially steals all the gas away from the black hole via gravity and as a result there is no matter to accrete and hence the system goes dark,” Pasham says.
It wasn’t immediately clear what caused the precipitous decline in the luminosity of AT2018fyk, because TDEs normally decay smoothly and gradually – not abruptly – in their emission. But around 600 days after the drop, the source was again found to be X-ray bright. This led the researchers to propose that the star survived its close encounter with the SMBH the first time and was in orbit about the black hole.
Using detailed modeling, the team’s findings suggest that the orbital period of the star about the black hole is roughly 1,200 days, and it takes approximately 600 days for the material that is shed from the star to return to the black hole and start accreting. Their model also constrained the size of the captured star, which they believe was about the size of the sun. As for the original binary, the team believes the two stars were extremely close to one another before being ripped apart by the black hole, likely orbiting each other every few days.
So how could a star survive its brush with death? It all comes down to a matter of proximity and trajectory. If the star collided head-on with the black hole and passed the event horizon – the threshold where the speed needed to escape the black hole surpasses the speed of light – the star would be consumed by the black hole. If the star passed very close to the black hole and crossed the so-called "tidal radius" – where the tidal force of the hole is stronger than the gravitational force that keeps the star together – it would be destroyed. In the model they have proposed, the star's orbit reaches a point of closest approach that is just outside of the tidal radius, but doesn't cross it completely: some of the material at the stellar surface is stripped by the black hole, but the material at its center remains intact.
How, or if, the process of the star orbiting the SMBH can occur over many repeated passages is a theoretical question that the team plans to investigate with future simulations. Syracuse physicist Eric Coughlin explains that they estimate between 1 to 10% of the mass of the star is lost each time it passes the black hole, with the large range due to uncertainty in modeling the emission from the TDE.
“If the mass loss is only at the 1% level, then we expect the star to survive for many more encounters, whereas if it is closer to 10%, the star may have already been destroyed,” notes Coughlin.
The team will keep their eyes to the sky in the coming years to test their predictions. Based on their model, they forecast that the source will abruptly disappear around August 2023 and brighten again when the freshly stripped material accretes onto the black hole in 2025.
The team says their study offers a new way forward for tracking and monitoring follow-up sources that have been detected in the past. The work also suggests a new paradigm for the origin of repeating flares from the centers of external galaxies.
“In the future, it is likely that more systems will be checked for late-time flares, especially now that this project puts forth a theoretical picture of the capture of the star through a dynamical exchange process and the ensuing repeated partial tidal disruption,” says Coughlin. “We’re hopeful this model can be used to infer the properties of distant supermassive black holes and gain an understanding of their “demographics,” being the number of black holes within a given mass range, which is otherwise difficult to achieve directly.”
The team says the model also makes several testable predictions about the tidal disruption process, and with more observations of systems like AT2018fyk, it should give insight into the physics of partial tidal disruption events and the extreme environments around supermassive black holes.
“This study outlines methodology to potentially predict the next snack times of supermassive black holes in external galaxies,” says Pasham. “If you think about it, it is pretty remarkable that we on Earth can align our telescopes to black holes millions of light years away to understand how they feed and grow.”
Read the full paper: https://ui.adsabs.harvard.edu/abs/2022arXiv220907538W/abstract
This illustration depicts a star (in the foreground) experiencing spaghettification as it’s sucked in by a supermassive black hole (in the background) during a ‘tidal disruption event’.
CREDIT
ESOM Kornmesser
Additional co-authors include: M. Guolo, Department of Physics and Astronomy, Johns Hopkins University; Y. Sun, University of Arizona; S. Wen, Department of Astrophysics/IMAPP, Radboud University ; P.G. Jonker, Department of Astrophysics/IMAPP, Radboud University and SRON, Netherlands Institute for Space Research ; A. Zabludoff, University of Arizona; A. Malyali, R. Arcodia, Z. Liu, A. Merloni, A. Rau and I. Grotova, Max-Planck-Institut fu ̈r extraterrestrische Physik , Germany; P. Short, Institute for Astronomy, University of Edinburgh; and Z. Cao, Department of Astrophysics/IMAPP, Radboud University
Video: https://youtu.be/_TRtPDbaQ2k
Animation describing a partial tidal disruption event – where a black hole repeatedly destroys a star.
JOURNAL
The Astrophysical Journal Letters
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
The American Astronomical Society, find out more The Institute of Physics, find out more THE FOLLOWING ARTICLE ISOPEN ACCESS Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event
DONUTIZATION
Hubble finds hungry black hole twisting captured star into donut shape
IMAGE: THIS SEQUENCE OF ARTIST'S ILLUSTRATIONS SHOWS HOW A BLACK HOLE CAN DEVOUR A BYPASSING STAR. 1. A NORMAL STAR PASSES NEAR A SUPERMASSIVE BLACK HOLE IN THE CENTER OF A GALAXY. 2. THE STAR'S OUTER GASSES ARE PULLED INTO THE BLACK HOLE'S GRAVITATIONAL FIELD. 3. THE STAR IS SHREDDED AS TIDAL FORCES PULL IT APART. 4. THE STELLAR REMNANTS ARE PULLED INTO A DONUT-SHAPED RING AROUND THE BLACK HOLE, AND WILL EVENTUALLY FALL INTO THE BLACK HOLE, UNLEASHING A TREMENDOUS AMOUNT OF LIGHT AND HIGH-ENERGY RADIATION. view more
CREDIT: NASA, ESA, LEAH HUSTAK (STSCI)
Black holes are gatherers, not hunters. They lie in wait until a hapless star wanders by. When the star gets close enough, the black hole's gravitational grasp violently rips it apart and sloppily devours its gasses while belching out intense radiation.
Astronomers using NASA's Hubble Space Telescope have recorded a star's final moments in detail as it gets gobbled up by a black hole.
These are termed "tidal disruption events." But the wording belies the complex, raw violence of a black hole encounter. There is a balance between the black hole's gravity pulling in star stuff, and radiation blowing material out. In other words, black holes are messy eaters. Astronomers are using Hubble to find out the details of what happens when a wayward star plunges into the gravitational abyss.
Hubble can't photograph the AT2022dsb tidal event's mayhem up close, since the munched-up star is nearly 300 million light-years away at the core of the galaxy ESO 583-G004. But astronomers used Hubble's powerful ultraviolet sensitivity to study the light from the shredded star, which include hydrogen, carbon, and more. The spectroscopy provides forensic clues to the black hole homicide.
About 100 tidal disruption events around black holes have been detected by astronomers using various telescopes. NASA recently reported that several of its high-energy space observatories spotted another black hole tidal disruption event on March 1, 2021, and it happened in another galaxy. Unlike Hubble observations, data was collected in X-ray light from an extremely hot corona around the black hole that formed after the star was already torn apart.
"However, there are still very few tidal events that are observed in ultraviolet light given the observing time. This is really unfortunate because there's a lot of information that you can get from the ultraviolet spectra," said Emily Engelthaler of the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Massachusetts. "We're excited because we can get these details about what the debris is doing. The tidal event can tell us a lot about a black hole." Changes in the doomed star's condition are taking place on the order of days or months.
For any given galaxy with a quiescent supermassive black hole at the center, it's estimated that the stellar shredding happens only a few times in every 100,000 years.
This AT2022dsb stellar snacking event was first caught on March 1, 2022 by the All-Sky Automated Survey for Supernovae (ASAS-SN or "Assassin"), a network of ground-based telescopes that surveys the extragalactic sky roughly once a week for violent, variable, and transient events that are shaping our universe. This energetic collision was close enough to Earth and bright enough for the Hubble astronomers to do ultraviolet spectroscopy over a longer than normal period of time.
"Typically, these events are hard to observe. You get maybe a few observations at the beginning of the disruption when it's really bright. Our program is different in that it is designed to look at a few tidal events over a year to see what happens," said Peter Maksym of the CfA. "We saw this early enough that we could observe it at these very intense black hole accretion stages. We saw the accretion rate drop as it turned to a trickle over time."
The Hubble spectroscopic data are interpreted as coming from a very bright, hot, donut-shaped area of gas that was once the star. This area, known as a torus, is the size of the solar system and is swirling around a black hole in the middle.
"We're looking somewhere on the edge of that donut. We're seeing a stellar wind from the black hole sweeping over the surface that's being projected towards us at speeds of 20 million miles per hour (three percent the speed of light)," said Maksym. "We really are still getting our heads around the event. You shred the star and then it's got this material that's making its way into the black hole. And so you've got models where you think you know what is going on, and then you've got what you actually see. This is an exciting place for scientists to be: right at the interface of the known and the unknown."
The results were reported at the 241st meeting of the American Astronomical Society in Seattle, Washington.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
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