In a groundbreaking breakthrough, a team of astronomers from MIT, NASA, and other institutions has successfully measured the spin of a supermassive black hole for the first time. Their innovative method involves analyzing the wobbly aftermath of a black hole's feasting on stellar material.

During a black hole tidal disruption event (TDE), when a passing star is torn apart by the immense tidal forces of a black hole, an accretion disk of rotating stellar material is generated. The MIT-led team discovered that the wobbling motion of this newly formed disk provides crucial information about the central black hole's spin.

The team's study, published in Nature, details how they tracked the pattern of X-ray flashes produced by the black hole immediately following the TDE. Over several months, they observed that these flashes were indicative of a wobbling accretion disk, influenced by the black hole's own spin. By studying the changes in the disk's wobble, the scientists were able to determine the rotational speed of the black hole.

Their analysis revealed that the measured supermassive black hole was spinning at less than 25 percent the speed of light, relatively slow compared to other black holes. This new method holds promise for estimating the spins of numerous black holes in the local universe in the coming years, providing insights into their evolution over time.

Lead author Dheeraj "DJ" Pasham, a research scientist at MIT's Kavli Institute for Astrophysics and Space Research, suggests that by studying multiple systems with this method, astronomers can gain an understanding of the distribution and evolution of black hole spins.

Black holes acquire their inherent spin through cosmic encounters. If a black hole predominantly grows through accretion, its spin increases at high speeds. Conversely, if black holes grow primarily through mergers with other black holes, these mergers may slow down the spin as they interact.

The researchers leveraged the phenomenon of Lense-Thirring precession, where a spinning black hole drags the surrounding space-time. Although this effect is typically imperceptible due to the lack of emitted light by black holes, scientists theorized that during a TDE, they may have a chance to observe the light emitted by the disrupted stellar debris as it is dragged around, enabling measurement of the black hole's spin.

During a TDE, a star falling onto a black hole generates a misaligned disk of shredded material, similar to a tilted spinning donut. As the disk encounters the black hole's spin, it wobbles until it aligns with the black hole's rotation. The researchers anticipated that monitoring the wobbling disk would provide a measurable signature of the black hole's spin.

The successful observation of such a wobbling disk required continuous, long-term observations. Pasham and his team capitalized on NASA's NICER telescope, stationed on the International Space Station, which provided high-cadence X-ray observations over 200 days following the initial detection of the TDE event named AT2020ocn.

NICER's data revealed periodic X-ray peaks occurring every 15 days, suggesting that the accretion disk wobbled face-on before moving away and emitting X-rays (analogous to waving a flashlight back and forth). Integrating this wobbling pattern into the Lense-Thirring precession theory, the team estimated the black hole's spin to be less than 25 percent the speed of light.

The findings mark a major achievement as scientists have used observations of a wobbling disk following a TDE to estimate the spin of a black hole for the first time. With the imminent launch of new telescopes like the Rubin Observatory, Pasham anticipates further opportunities to unlock the secrets of black hole spins.

Understanding the spin of supermassive black holes provides valuable insights into their cosmic history. Should future observations capture similar signals from hundreds of TDEs, scientists will be equipped to make significant discoveries about the evolution of black holes throughout the age of the universe.

Reference: Dheeraj Pasham, Lense-Thirring precession after a supermassive black hole disrupts a star, Nature (2024). DOI: 10.1038/s41586-024-07433-w. www.nature.com/articles/s41586-024-07433-w.