Black hole spin speed revealed in new study of churning space-time

The "wobbling" remains of a star that suffered a grisly death at the maw of a supermassive black hole has helped reveal the speed at which its cosmic predator spins.

Supermassive black holes are believed to be born through successive mergers of smaller black holes, each of which brings with it angular momentum that speeds up the rotation of the black hole they birth. Consequently, measuring the spin of supermassive black holes can grant insight into their History — and new research offers a new way to make such inferences based on the effect spinning black holes have on the very fabric of space and time.

The doomed star at the heart of this research was ripped apart in brutal fashion by a supermassive black hole during a so-called tidal disruption event (TDE). These events are kicked off when a star ventures too close to the massive gravitational influence of a black hole. Once close enough, immense tidal forces are generated within the star, which squash it horizontally while stretching it vertically. That's called "spaghettification," and its a process that turns the star into a strand of stellar pasta — but, crucially, not all of it is gobbled by the destructive black hole.

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Some of this material is blown away, while some of it wraps around the black hole, forming a flattened cloud called an accretion disk. Not only does this accretion disk gradually feed the central black hole, but the same tidal forces that shredded the star in the first place also cause massive friction forces that heat this platter of gas and dust, causing it to glow brightly. 

Furthermore, when supermassive black holes spin, they drag along with them the very fabric of spacetime (a 4-dimensional unity of space and time). This so-called "Lense-Thirring" or "frame-dragging" effect means nothing stands still at the edge of a spinning supermassive black hole. The effect also causes a short-lived "wobble" in a newly formed black hole accretion disk.

Now, a team of researchers has discovered that the "wobble" of that accretion disk can be used to determine how fast the central black hole is spinning.