Nature: Ultrafast rotation of a supermassive black hole measured for the first time
Astronomers at MIT, NASA and others have measured the ultra-fast spin of a supermassive black hole for the first time, watching as it bends the space around it. Research results published in the journal Nature.
The method is based on tidal disruption, when a star approaching a black hole is torn into pieces. About half the star’s mass is carried away by the cosmic wind, while the other forms an extremely hot accretion disk. Scientists have shown that the oscillations (precession) of the disk, which can be tracked by X-ray flares, are associated with the rotation of the black hole.
In February 2020, scientists discovered AT2020ocn, a bright flare coming from a galaxy about a billion light-years away from Earth. It occurred in the first moments after the tidal disruption event. The observations were made with NASA’s NICER X-ray telescope for 200 days after the flare. X-rays were detected that peaked every 15 days for several cycles before disappearing.
The rotation of a black hole bends the space-time surrounding it, resulting in additional accelerations for bodies moving in the gravitational field. This effect, predicted by general relativity, is known as drag of inertial frames or the Lense-Thirring effect. It is difficult to observe near black holes because these objects do not emit light, but it can be visible if an accretion disk is present.
Astronomers tracked how the disk’s wobbles changed over time and then incorporated the findings into the Lense-Thirring theory of precession. Based on an estimate of the mass of the black hole and the destroyed star, scientists found that the black hole was spinning at less than 25 percent of the speed of light, which is relatively slow compared to other black holes.
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