by The “wobbling” remains of a star that suffered an ugly death at the maw of a supermassive black hole helped reveal the speed of its spinning cosmic predator.
Supermassive black holes are believed to be born through successive mergers of smaller black holes, each of which adds angular momentum that speeds up the spin of the parent black hole. As a result, the spin of supermassive black holes can be measured to provide insight into their history – and new research offers a new way to make such inferences based on the effect that spinning black holes have on the true shape of the space and time.
The doomed star at the heart of this research was violently torn apart by a supermassive black hole during a so-called tidal disruption event (TDE). These events are initiated when a star passes too close to the gravitational influence of a massive black hole. When it is close enough, enormous tidal forces are generated within the star, which squash it horizontally and stretch it vertically. This is called “spaghettiation”, and it’s a process that turns the star into a layer of interstellar pasta – but, crucially, the destructive black hole is not completely obliterating it.
Some of this material is blown away, and some of it wraps around the black hole, forming a flattened cloud called an accretion disk. Not only does this accretion disk gradually feed the central black hole, but the same tidal forces that destroyed the star in the first place cause enormous frictional forces to heat this platter of gas and dust, which makes it shine brightly.
Related: Scientists discover a spaghettig star black hole extremely close to Earth
Furthermore, when supermassive black holes spin, they drag with them the fabric of space-time (the 4-dimensional unity of space and time). The so-called “Lense-Thirring” or “frame pulling” effect means that nothing remains at the edge of a spinning supermassive black hole. The effect also creates a short-lived “wobble” in the accretion disk of a newly formed black hole.
Now, a team of researchers has discovered that the “wobble” of that accretion disk can be used to determine how fast the central black hole is spinning.
“It’s a frame dragging effect around all spinning black holes,” team leader Dheeraj “DJ” Pasham, a scientist at the Massachusetts Institute of Technology (MIT), told Space.com. “Therefore, if the black hole is turbulently spinning, then the flow of stellar debris into the black hole after a TDE will be subject to this effect.”
Hot holy X-ray stellar pasta!
To investigate TDEs and frame-dragging, the team spent five years searching for bright and relatively close examples of stellar murders that could be followed quickly. The goal was to find signs of an accretion disk precursor resulting from the Lense-Thirring effect.
In February of 2020, this search came to fruition. The team managed to detect AT2020ocn, a bright flash of light coming from a galaxy located about a billion light years away. AT2020ocn was first seen in optical wavelengths at the Zwicky Transient Facility, and this visible light data shows the emission from a TDE containing a supermassive black hole with a mass between 1 million and 10 million times the mass of the Sun .
“Because of the Lense-Thirring effect, the X-ray emission coming from the newly formed, hot disks and precursors, or ‘wobbles.’ This shows up as X-ray modifications in the data,” said Pasham. “However, after a while, when the accretion power comes down, gravity forces the disk to align with the black hole, and subsequently the wobbling and X-ray modifications stop.”
Pasham and colleagues suspected that the TDE sent by AT2020ocn might be the ideal event to hunt down the Lense-Thirring precursor – and because this type of wobble is only present soon after the formation of an accretion disc, they had to act quickly.
“The key was to have the right views,” Pasham said. “The only way you can do this is, once a tidal disruption occurs, you need to get a telescope to look at this object continuously, for a very long time, so that you can explore all kinds of time scales, from minutes. to months.”
That’s where NASA’s Neutron Star Interior Composition ExploreR (NICER) comes in: an X-ray telescope on the International Space Station (ISS) that measures X-ray radiation around black holes and other ultradense, dense massive objects like neutron stars. The team found that not only was NICER able to capture the TDE, but the X-ray telescope on the ISS was able to continuously monitor the event as it developed over several months.
“We found that the X-ray brightness and temperature of the X-ray emitting region after a TDE is modulated on a time scale of 15 days,” said Pasham. “This 15-day recurring X-ray sign disappeared after three months.”
The team’s results also surprised people.
Estimates of the mass of the black hole and the mass of the disturbed star showed that the black hole was not spinning as fast as expected. “It was a little surprising that the black hole is not spinning that fast – just less than 25% of the speed of light,” said Pasham.
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Pasham thinks that thanks to the upcoming Vera C. Rubin Observatory, currently under construction in northern Chile and planned for a 10-year survey of the universe called the Legacy Space and Time Survey (LSST) , the future is bright for TDE. – debt.
“Rubin is expected to find tens of thousands of TDEs over the next decade. If we can measure Lense-Thirring precursors in even a small fraction of them, we will be able to say something about the spin distribution of supermassive black holes, which with. how they evolved over the ages,” Pasham concluded. “Our team has observed a few proposals to follow up with TDEs in the future. We will definitely be exploring frame-trapping around other TDE black holes!”
The team’s research was published on Wednesday (May 22) in the journal Nature.
