The “shaking” remains of a star that suffered a grisly death at the maw of a supermassive black hole have helped reveal the spin rate of the cosmic predator.
Supermassive black holes are believed to be born through successive mergers of smaller black holes; Each of these brings with it angular momentum that accelerates the spin of the black hole they gave birth to. As a result, measuring the spin of supermassive black holes can provide insight into their history, and new research offers a new way to make such inferences based on the impact of spinning black holes on the fabric of space and time.
The doomed star at the heart of this research was brutally ripped apart by a supermassive black hole during an event called a tidal disruption event (TDE). These events begin when a star comes too close to the massive gravitational pull of a black hole. When it gets close enough, tremendous tidal forces build up inside the star, compressing it horizontally and stretching it vertically. This is called “spaghettification” and is a process that transforms the star into a strand of star pasta; But most importantly, it is not completely swallowed by the destructive black hole.
Some of this material flies away, and some surrounds the black hole, forming a flattened cloud called an accretion disk. Not only does this accretion disk slowly feed the central black hole, but the tidal forces that tear the star apart also cause massive frictional forces that heat this layer of gas and dust, causing it to glow brightly.
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Moreover, when supermassive black holes spin, they drag the fabric of space-time (the 4-dimensional unity of space and time) with them. This effect, called the “Lense-Thirring” or “frame drag” effect, means that nothing stands still at the edge of a spinning supermassive black hole. This effect also causes a brief “wobble” in a newly formed black hole accretion disk.
Now a research team has discovered that the “wobble” of this accretion disk can be used to determine how fast the central black hole is spinning.
“Frame drag is an effect present around all rotating black holes,” team leader Dheeraj “DJ” Pasham, a scientist at the Massachusetts Institute of Technology (MIT), told Space.com. “So, if the disruptive black hole is rotating, then the flow of stellar debris into the black hole following a TDE is subject to this effect.”
Holy hot X-ray star pasta!
To investigate TDEs and frame dragging, the team spent five years searching for bright and relatively close examples of stellar murders caused by black holes that could be quickly tracked. The aim was to detect signatures of accretion disk precession caused by the Lense-Thirring effect.
In February 2020, this search came to fruition. The team managed to detect AT2020ocn, a bright flash of light coming from a galaxy about a billion light-years away. AT2020ocn was first detected in optical light wavelengths by the Zwicky Transient Facility; These visible light data indicate that the emission originates from a TDE containing a supermassive black hole with a mass between 1 million and 10 million times that of the Sun.
“X-ray emission from newly formed hot accretion disk precessions, or ‘wobbles’, due to the Lense-Thirring effect. This manifests as X-ray modulations in the data,” said Pasham. “But after a while, when the accretion strength decreases, gravity forces the disk to align with the black hole, after which the wobble and X-ray modulations stop.”
Pasham and his colleagues suspected that the TDE that initiated AT2020ocn might be an ideal event to hunt for Lense-Thirring precession, and they had to act quickly because such wobbles only appear soon after an accretion disk forms.
“The important thing was to have accurate observations,” Pasham said. “The only way to do this is as soon as a tidal disruption event occurs, you need to get a telescope to look at that object continuously over a very long period of time, so you can study all kinds of time scales, from minutes to months.”
That’s where NASA’s Neutron Star Internal Composition Research (NICER) comes in: an X-ray telescope aboard the International Space Station (ISS) that measures X-ray radiation around black holes and other ultradense, compact large objects such as neutron stars. The team found that not only did NICER capture the TDE, but the ISS-mounted X-ray telescope was also able to continuously monitor the evolving event over several months.
“We discovered that following a TDE, the X-ray brightness and the temperature of the X-ray emitting region are modulated on a 15-day time scale,” said Pasham. “This recurring 15-day X-ray signal disappeared after three months.”
The team’s findings also created a surprise.
Estimates of the mass of the black hole and the mass of the disintegrating star revealed that the black hole was not spinning as fast as expected. “It was a little surprising that the black hole wasn’t spinning that fast; only less than 25% of the speed of light,” Pasham said. said.
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Pasham thinks the future of TDE is bright, thanks to the Vera C. Rubin Observatory, currently under construction in northern Chile, which will conduct a 10-year survey of the universe called the Survey of Ancient Space and Time (LSST). -hunt.
“Rubin is expected to detect thousands of TDEs over the next decade. If we can measure the Lense-Thirring precession in even a small fraction of them, we will be able to say something about the spin distribution of supermassive black holes. Pasham explained how they have evolved over the age of the universe. “Our team’s future There are several monitoring recommendations lined up to track TDEs. We will definitely be investigating frame dragging around other TDE black holes!”
The team’s research was published in the journal Nature on Wednesday, May 22.