Researchers have determined the spin speed of a supermassive black hole by examining the ‘spaghettified’ remnants of a star it devoured.
Black holes can disintegrate stars in a process known as a tidal disruption event (TDE). By studying the aftermath of such an event, scientists revealed the rotational speed of the supermassive black hole responsible. Supermassive black holes likely form from the merging of smaller black holes, each contributing angular momentum that accelerates the spin of the resulting black hole. Therefore, measuring this spin can provide insights into the black hole’s history.
In this study, a star was destroyed by a supermassive black hole during a TDE, where the star was stretched vertically and compressed horizontally—an effect known as “spaghettification.” The remnants formed an accretion disk around the black hole, heating up and glowing due to intense friction forces. This disk’s formation is influenced by the black hole’s spin, which drags space-time around it in a phenomenon called “frame-dragging” or the “Lense-Thirring effect.” This effect causes a temporary “wobble” in the accretion disk.
A team of researchers found that this wobble could be used to gauge the black hole’s spin rate. They focused on detecting signs of accretion disk precession caused by frame-dragging. In February 2020, they observed AT2020ocn, a bright light from a galaxy a billion light-years away, indicating a TDE involving a supermassive black hole with a mass between 1 million and 10 million times that of the sun.
NASA’s Neutron Star Interior Composition ExploreR (NICER), an X-ray telescope on the International Space Station, monitored the TDE continuously. It detected a recurring 15-day X-ray modulation, which ceased after three months. This observation helped estimate the black hole’s spin, which was surprisingly slow—less than 25% the speed of light.
Future advancements in TDE studies are anticipated with the upcoming Vera C. Rubin Observatory, expected to detect thousands of TDEs over the next decade. This will enable further investigation into the spin distribution of supermassive black holes and their evolutionary history.
