Cosmic Whirlpools: Unveiling the Mysteries of a Spinning Black Hole

Monday 27 May 2024 - 16:50

In a groundbreaking achievement, a team of astronomers led by astrophysicist Dheeraj Pasham from the prestigious Massachusetts Institute of Technology (MIT) has successfully calculated the rotational speed of a distant supermassive black hole, situated a staggering one billion light-years away from Earth. This feat was accomplished through the use of a novel technique that meticulously observed the oscillations of the black hole's accretion disk.

These oscillations are the periodic variations in density, temperature, and other physical parameters of the swirling material that encircles the black hole before plunging into its insatiable depths. The remarkable findings of this research were published on May 22 in the esteemed scientific journal, *Nature*.

A Dizzying Endeavor

By scrutinizing the subtle fluctuations of light emitted during the awakening of a low-activity black hole, the scientists were able to determine its rotational velocity, a first-of-its-kind achievement. The result? This cosmic colossus spins at an estimated speed of less than 25% of the speed of light, approximately 74,948 km/s. As Pasham, one of the researchers involved in the study, elucidates, "By studying multiple systems in the coming years with this method, astronomers will be able to estimate the overall distribution of black hole spin rates and understand how they evolve over time."

These celestial titans, nestled at the hearts of galaxies, can transition from quiescence to intense activity, producing some of the brightest outbursts in the universe. In 2020, astronomers witnessed the spectacular awakening of a dormant black hole in a distant galaxy, igniting a colossal burst of light dubbed AT2020ocn. Data revealed that this intense light emission was triggered by an event known as a "tidal disruption event," where a star is devoured by the black hole's gravitational pull, forming a luminous disk around it.

A Meticulous Observation

To accurately measure the black hole's rotation, it was essential to observe this event from its inception. Pasham explains, "To succeed, you must immediately point a telescope at the object as soon as a tidal disruption event occurs and observe it continuously for an extended period, in order to analyze different timescales, ranging from minutes to several months."

With telescopes continuously scanning the skies, the researchers were able to capture AT2020ocn just in time. They observed that the galaxy emitted X-rays approximately every 15 days, a phenomenon they linked to the oscillation of the accretion disk surrounding the nearby black hole. By combining these observations with the estimated mass of the black hole (approximately 2.5 million times the mass of the Sun), they were able to precisely calculate its rotational speed.

This novel measurement technique could radically transform our understanding of black holes. It is far more precise and direct than the indirect observational methods previously employed. By analyzing the luminosity variations of the accretion disk, it is now possible to obtain more reliable and exploitable data in a wider range of situations, such as disk evolution, interactions between the black hole and nearby stars, or periods of low activity. Thanks to this discovery, it will one day be possible to map the various rotational speeds of black holes, enabling us to better comprehend the evolution of these cosmic objects across the ages.

The MIT researchers have successfully calculated the rotational speed of a black hole through a groundbreaking new method. Rather than observing it indirectly, they analyzed the luminosity variations of its accretion disk, a technique that will allow for the collection of more reliable data on black holes and enhance our understanding of these enigmatic cosmic phenomena.