Welsh university’s scientists help confirm century-old black hole theory

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A global team of scientists including those from a Welsh university have seen a breakthrough in their search for one of the most elusive phenomena in the night sky.

The team is led by the National Astronomical Observatories at the Chinese Academy of Sciences and supported by Cardiff University.

They examined a tidal disruption event (TDE), named AT2020afhd, where a star was torn apart by a supermassive black hole.

Their study, presented today in Science Advances, reports on the very first observations of a swirling vortex in spacetime caused by a rapidly rotating black hole.

The process, known as Lense-Thirring precession or frame-dragging, describes how black holes twist the spacetime that surrounds them, dragging nearby objects like stars and wobbling their orbits along the way.

A swirling disk formed around the black hole made up of the star’s leftovers, from which powerful jets of matter shot out at nearly the speed of light.

Through rhythmic changes in both X-ray and radio signals coming from the event, the team observed the disk and the jet were wobbling in unison, repeating every 20 days.

First theorised by Einstein in 1913 and then mathematically defined by Lense and Thirring in 1918, the observation confirms a general relativity prediction and offers scientists new avenues for studying black hole spin, accretion physics, and jet formation.

Dr Cosimo Inserra, a Reader in the School of Physics and Astronomy at Cardiff University and one of the paper’s co-authors, said: “Our study shows the most compelling evidence yet of Lense-Thirring precession – a black hole dragging space time along with it in much the same way that a spinning top might drag the water around it in a whirlpool.

“This is a real gift for physicists as we confirm predictions made more than a century ago. Not only that, but these observations also tell us more about the nature of TDEs – when a star is shredded by the immense gravitational forces exerted by a black hole.

“Unlike previous TDEs studied, which have steady radio signals, the signal for AT2020afhd showed short-term changes, which we were unable to attribute to the energy release from the black hole and its surrounding components. This further confirmed the dragging effect in our minds and offers scientists a new method for probing black holes.”

The team modelled X-ray data from the Neil Gehrels Swift Observatory (Swift) and radio signal data from the Karl G. Jansky Very Large Array (VLA) to identify the frame dragging effect.

Further analysis of the composition, structure and properties of the cosmic matter with electromagnetic spectroscopy enabled them to describe and identify the process.

“By showing that a black hole can drag space time and create this frame-dragging effect, we are also beginning to understand the mechanics of the process,” explains Dr Inserra.

“So, in the same way a charged object creates a magnetic field when it rotates, we’re seeing how a massive spinning object – in this case a black hole – generates a gravitomagnetic field that influences the motion of stars and other cosmic objects nearby.

“It’s a reminder to us, especially during the festive season as we gaze up at the night sky in wonder, that we have within our grasp the opportunity to identify ever more extraordinary objects in all the variations and flavours that nature has produced.”

The paper, ‘Detection of disk–jet coprecession in a tidal disruption event’, is published in Science Advances.