Most black holes may spin far more slowly than expected. When black holes form from collapsing stars, they emit powerful jets of energy, and these jets slow down the black holes鈥� spin to nearly nothing within minutes.
The spin of a newborn black hole is governed by two competing phenomena. When fast-moving material in its orbit falls in, the black hole takes in energy and its rotation speeds up. But that same consumption causes a huge jet to blast away from the black hole for a few minutes, which leaks away energy and slows it down.
at Northwestern University in Illinois and his colleagues used a series of simulations to examine how these two processes interact and where that leaves black holes once their formation is complete. They found that collapsars 鈥� the name given to black holes left behind after a star collapses, which make up most of the black holes in the cosmos 鈥� should barely be spinning at all.
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鈥淏efore, most works didn鈥檛 really consider the spin-down through the jet, so [a simulated black hole] would just eat and eat and eat the gas and that would spin it up more and more, so you would expect a lot of black holes to be maximally spinning,鈥� says Jacquemin-Ide.
Astronomers measure black hole rotation using a parameter called the dimensionless spin, or a. When a equals 0, the black hole is not spinning at all. When a equals 1, it is spinning at the maximum possible rate for an object with its mass. The researchers found that just after their births, collapsars should rotate with a dimensionless spin of about 0.2 or lower.
鈥淚t shows just how powerful the jets can be,鈥� says Jacquemin-Ide. 鈥淲hat surprised me most is that this happens very fast 鈥� you have this event that lasts maybe 100 seconds, but it can absolutely change the spin of the black hole.鈥�
This matches what astronomers have observed through gravitational waves, which are ripples in space-time that occur due to the motion of massive objects. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has felt the waves from nearly 100 mergers between pairs of black holes, and measurements indicate that the dimensionless spins of the black holes before they slam together tend to be around 0.2.
鈥淲ith gravitational waves, we observe black holes at a very specific instance in their lives, right at the end, and they spend billions of years before that doing other stuff,鈥� says at the California Institute of Technology. 鈥淭his work adds one more piece to the puzzle of their lifetimes.鈥�
It could also help us distinguish between different types of black holes. 鈥淭here are other ways than stellar collapse to make black holes 鈥� perhaps more rare, but they are there,鈥� says Chatziioannou. 鈥淥ne of the big issues for us is how to tell them apart, and this might be a way to separate them.鈥�
Black holes that have undergone mergers tend to have higher spins, closer to 0.7, than the 鈥減ristine鈥� ones the researchers considered here, so measuring the spin of a black hole could help us piece together its past.
The Astrophysical Journal
