An Unlucky Star’s Fatal Encounter with a Black Hole
When a star ventures too close to a black hole, gravitational forces create intense tides that break the star apart into a stream of gas, resulting in a cataclysmic phenomenon known as a Tidal Disruption Event (TDE). Tremendous amounts of energy are released, causing a tidal disruption to outshine its galaxy in some cases. This illustration shows a glowing stream of material from a star, torn to shreds as it was being devoured by a black hole. The feeding black hole is surrounded by a ring of dust. (Content Credit: Daniel Stolte, University of Arizona) (Image Credit: NASA, Chandra X-Ray Center - CXC, M.Weiss)
An Unlucky Star’s Fatal Encounter with a Black Hole
Analyzing observations of an X-ray flare and fitting the data with theoretical models, University of Arizona astronomers have documented a fatal encounter between an unlucky star and an intermediate-mass black hole.
When a black hole gobbles up a star, it produces what astronomers call a Tidal Disruption Event (TDE). The shredding of the hapless star is accompanied by an outburst of radiation that can outshine the combined light of every star in the black hole's host galaxy for months, even years.
A team of astronomers led by Sixiang Wen, a postdoctoral research associate at the University of Arizona Steward Observatory, used the X-rays emitted by a tidal disruption event known as J2150 to make the first measurements of both the black hole's mass and spin. This black hole is of a particular type – an intermediate-mass black hole – which has long eluded observation.
"The fact that we were able to catch this black hole while it was devouring a star offers a remarkable opportunity to observe what otherwise would be invisible," said Ann Zabludoff, University of Arizona professor of astronomy. "Not only that, by analyzing the flare we were able to better understand this elusive category of black holes, which may well account for the majority of black holes in the centers of galaxies."
By re-analyzing the X-ray data used to observe the J2150 flare and comparing it with sophisticated theoretical models, the researchers showed that this flare did indeed originate from an encounter between an unlucky star and an intermediate-mass black hole. The intermediate black hole in question is of particularly low mass (for a black hole, that is) weighing in at roughly 10,000 times the mass of the sun.
"The X-ray emissions from the inner disk formed by the debris of the dead star made it possible for us to infer the mass and spin of this black hole and classify it as an intermediate black hole," Wen said.
Dozens of tidal disruption events have been seen in the centers of large galaxies hosting supermassive black holes, and a handful have also been observed in the centers of small galaxies that might contain intermediate black holes. However, past data has never been detailed enough to prove that an individual tidal disruption flare was powered by an intermediate black hole.
"Thanks to modern astronomical observations, we know that the centers of almost all galaxies that are similar to or larger in size than our Milky Way host central supermassive black holes," said Nicholas Stone, a senior lecturer at Hebrew University in Jerusalem. "These behemoths range in size from 1 million to 10 billion times the mass of our sun, and they become powerful sources of electromagnetic radiation when too much interstellar gas falls into their vicinity."
The mass of these black holes correlates closely with the total mass of their host galaxies; the largest galaxies host the largest supermassive black holes.
"We still know very little about the existence of black holes in the centers of galaxies smaller than the Milky Way," said Peter Jonker of Radboud University and SRON Netherlands Institute for Space Research, both in the Netherlands. "Due to observational limitations, it is challenging to discover central black holes much smaller than 1 million solar masses."
Despite their presumed abundance, the origins of supermassive black holes remain unknown, and many different theories currently vie to explain them, according to Jonker. Intermediate-mass black holes could be the seeds from which supermassive black holes grow.
"Therefore, if we get a better handle of how many bona fide intermediate black holes are out there, it can help determine which theories of supermassive black hole formation are correct," he said.
Even more exciting, according to Zabludoff, is the measurement of J2150's spin that the group was able to obtain. The spin measurement holds clues as to how black holes grow.
This black hole has a fast spin, but not the fastest possible spin, Zabludoff explained, begging the question of how the black hole ends up with a spin in this range.
"It's possible that the black hole formed that way and hasn't changed much since, or that two intermediate-mass black holes merged recently to form this one," she said. "We do know that the spin we measured excludes scenarios where the black hole grows over a long time from steadily eating gas or from many quick gas snacks that arrive from random directions."
In addition, the spin measurement allows astrophysicists to test hypotheses about the nature of dark matter, which is thought to make up most of the matter in the universe. Dark matter may consist of unknown elementary particles not yet seen in laboratory experiments. Among the candidates are hypothetical particles known as ultralight bosons, Stone explained.
"If those particles exist and have masses in a certain range, they will prevent an intermediate-mass black hole from having a fast spin," he said. "Yet J2150’s black hole is spinning fast. So, our spin measurement rules out a broad class of ultralight boson theories, showcasing the value of black holes as extraterrestrial laboratories for particle physics."
In the future, new observations of tidal disruption flares might let astronomers fill in the gaps in the black hole mass distribution.
"If it turns out that most dwarf galaxies contain intermediate-mass black holes, then they will dominate the rate of stellar tidal disruption," Stone said. "By fitting the X-ray emission from these flares to theoretical models, we can conduct a census of the intermediate-mass black hole population in the universe," Wen added.
To do that, however, more tidal disruption events have to be observed. That's why astronomers hold high hopes for new telescopes coming online soon, both on Earth and in space, including the Vera C. Rubin Observatory, also known as the Legacy Survey of Space and Time, or LSST, which is expected to discover thousands of tidal disruption events per year.
A Happier Ending – What Happens When a Lucky Star Meets a Black Hole and Escapes?
Back in 2014, astronomers got a close look at what happens when a lucky star meets a black hole and finds a way to escape. In this case, the black hole took a bite out of a star, but the star lived to tell the tale.
We may think of black holes as swallowing entire stars or any other objects that wander too close to their immense gravity. But sometimes, a star that is almost captured by a black hole escapes with only a portion of its mass torn off. Such was the case for a star some 650 million light years away toward Ursa Major, where a supermassive black hole tore off a chunk of material from a star that got away.
Astronomers at The Ohio State University couldn't see the star itself with their All-Sky Automated Survey for Supernovae (ASAS-SN, pronounced "Assassin"). But they did see the light that flared as the black hole ate the material that it managed to capture.
The astronomers report that the star and the black hole are located in a galaxy outside of the newly dubbed Laniakea Supercluster, of which our home Milky Way Galaxy is a part.
If Laniakea is our galactic "city," then this event, called a "Tidal Disruption Event," or TDE, happened in our larger metropolitan area. Still, it's the closest TDE ever spotted, and it gives astronomers the best chance yet of learning more about how supermassive black holes form and grow.
ASAS-SN has so far spotted more than 60 bright and nearby supernovae. One of the program's other goals is to try to determine how often TDEs happen in the nearby universe. But Krzysztof Stanek, professor of astronomy at Ohio State, and his collaborators were surprised to find one in January 2014, just a few months after ASAS-SN's four telescopes in Hawaii began gathering data.
To Stanek, the fact that the survey made such a rare find so quickly suggests that TDEs may be more common than astronomers realized.
"We found one right out of the gate," he said. "Based on that, we are encouraged that the rate may be higher than one TDE every year or two."
"You could say we just got lucky, but when you get lucky time after time, you're doing something right," he continued. "Maybe the rate truly is higher than people expected, which would mean that we should be seeing more of these in the near future."
Doctoral student Thomas Holoien led the observations and analysis of the TDE when it first flared to brightness on January 25, 2014. It appeared near the back left foot of Ursa Major, between the stars Alula Borealis and Praecipua. He labeled the object ASASSN-14ae, and at first he thought it was a supernova, albeit an unusual looking one. But its brightness pattern ultimately indicated something else and he and his colleagues determined that they were seeing a TDE.
They followed-up ground observations via the one meter robotic telescope at McDonald Observatory, the two meter robotic Liverpool Telescope, the 3.5 meter Apache Point Observatory telescope, and the double 8.4 meter Large Binocular Telescope. Then they added space-based observations from the Swift UltraViolet and Optical Telescope. Finally, they used Sloan Digital Sky Survey archival data to place the object within its host galaxy, called SDSS J110840.11.
Based on the amount of energy released during the event, the researchers calculated that a relatively small amount of stellar material -- only one thousandth of the mass of our sun, an amount approximately equal to the mass of the planet Jupiter -- had been sucked into the black hole.
Christopher Kochanek, Professor of Astronomy at Ohio State, was among the first experts to model TDEs 25 years ago, as a graduate student. Only a handful have been spotted since then, however, and nobody knows for sure how important a contribution TDEs might make to the growth of black holes in the universe.
"Conventional wisdom suggests that black holes don't consume whole stars all that often -- maybe only once every 10,000 to 100,000 years," Kochanek said. But how often black holes tear off just a piece of a passing star is an open question.
"The issue is that the chances of a black hole partly shredding a star may not be all that different from the chances of it completely shredding a star. We just don't know," Kochanek said.
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