The ‘wobbly’ remains of a star that died a gruesome death in the maw of a supermassive black hole have helped reveal the speed at which its cosmic predator spins.
Supermassive black holes are believed to form from successive mergers of smaller black holes, each of which brings with it an angular momentum that accelerates the rotation of the black hole they produce. Consequently, measuring the spin of supermassive black holes can provide insight into their history – and new research offers a new way to draw such conclusions based on the effect that spinning black holes have on the fabric of space and time.
The doomed star at the heart of this study was brutally torn apart by a supermassive black hole during a so-called Tidal Disruption Event (TDE). These events are triggered when a star moves too close to the enormous gravitational influence of a black hole. Once close enough, enormous tidal forces are generated in the star, flattening it horizontally and stretching it vertically. That’s called ‘spaghettification’, and it’s a process that turns the star into a stellar paste – but crucially not all of it gets swallowed up by the destructive black hole.
Some of this material is blown away, while some wraps around the black hole, forming an oblate cloud called an accretion disk. Not only does this accretion disk gradually feed the central black hole, but the same tidal forces that tore the star apart in the first place also create enormous frictional forces that heat this dish of gas and dust, causing it to glow brightly.
Related: Scientists find a spaghetti-fusing star with a black hole remarkably close to Earth
Furthermore, as supermassive black holes spin, they drag the fabric of spacetime (a four-dimensional unit of space and time) with them. This so-called ‘Lense-Thirring’ or ‘frame-dragging’ effect means that nothing stands still at the edge of a spinning supermassive black hole. The effect also causes a momentary “wobble” in a newly formed black hole accretion disk.
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.
“Frame dragging is an effect that is present around all rotating black holes,” team leader Dheeraj “DJ” Pasham, a scientist at the Massachusetts Institute of Technology (MIT), told Space.com. “So as the disruptive black hole rotates, the flow of the stellar debris into the black hole after a TDE is subject to this effect.”
Holy hot x-ray star paste!
To investigate TDEs and frame dragging, the team spent five years looking for bright and relatively good examples of black hole-induced stellar killings that could be followed up quickly. The aim was to detect signs 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 billion light years away. AT2020ocn was initially observed in optical light wavelengths by the Zwicky Transient Facility, with this visible light data indicating that the emission came from a TDE involving a supermassive black hole with a mass between 1 million and 10 million times that of the Sun .
“Because of the Lense-Thirring effect, the X-rays coming from the newly formed, hot accretion disk precess, or ‘wobble’. This manifests as gravity pulls the disk into alignment with the black hole, at which point the wobble and X-ray modulations stop.”
Pasham and colleagues suspected that the TDE that launched AT2020ocn could be the ideal event to drive the Lense-Thirring precession — and because this kind of wobble is only present shortly after an accretion disk forms, they had to act quickly.
“The key was to have the right observations,” Pasham said. “The only way you can do this is that once a tidal disruption happens, you need a telescope to look at this object continuously for a very long time, so you can examine all kinds of time scales, from minutes to 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 emissions around black holes and other ultra-dense, compact massive objects such as neutron stars. The team found that not only could NICER capture the TDE, but the ISS-mounted X-ray telescope could also continuously monitor the event for several months.
“We found that the X-ray brightness and temperature of the X-ray emitting region modulate after a TDE on a time scale of 15 days,” Pasham said. “This recurring X-ray signal of 15 days disappeared after three months.”
The team’s findings also provided a surprise.
Estimates of the black hole’s mass and the mass of the disrupted star revealed that the black hole was not spinning as fast as expected. “It was somewhat surprising that the black hole is not spinning that fast — only less than 25% of the speed of light,” Pasham said.
Related stories:
— Black hole announces itself to astronomers by violently tearing apart a star
– Record breaker! The newly discovered black hole is closest to Earth
— NASA X-ray observatory shows how black holes swallow stars and spit out matter
Pasham believes that, thanks to the upcoming Vera C. Rubin Observatory, currently under construction in northern Chile and which will conduct a ten-year survey of the universe, the Legacy Survey of Space and Time (LSST), the future looks bright for TDE -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 part of it, we will be able to say something about the spin distribution of supermassive black holes, associated with how they have evolved over the time of the universe’ , Pasham concluded. “Our team has prepared a number of observational proposals to monitor future TDEs. We will certainly investigate frame dragging around other TDE black holes!”
The team’s research was published Wednesday (May 22) in the journal Nature.