For black holes, going with the flow might make you stand out more. A recent paper by Alexander Tchekhovskoy (Princeton) and Jonathan McKinney (Stanford) suggests that a black hole gobbling infalling gas and dust will spit out a slightly more powerful jet if it spins the same direction as the disk of material it’s eating. The result, part of a larger effort to understand jets’ maximum energies, adds to an ongoing debate among astronomers over whether spin direction — and which direction — affects the outflow's power.
Astronomers already knew that a faster spin causes a more powerful jet, a conclusion borne out by recent observations, Tchekhovskoy explains. That’s because jets can steal energy from a spinning beast, and a larger spin has more energy. Tchekhovskoy and his colleagues found last year that a jet outflow can actually put out more energy than is available from the disk of material feeding it (remember, mass is energy, as Einstein famously showed). That energy comes from the black hole's spin.
“Imagine you invest $100 worth of mass-energy into a black hole and $20 worth of energy comes out in the form of the jets,” Tchekhovskoy explains. “This means that jet efficiency is 20%.” In the new study, for a black hole spinning the same way (prograde) as the disk, “we put in $100 and $105 comes back out — efficiency is 105%.” A black hole spinning the same rate but in the opposite direction only produced $38 — that is, it converted the accreting mass into the jet with 38% efficiency.
While the prograde spin produced a jet a few times more powerful than the black hole spinning backward, or retrograde, the difference is probably not enough to allow astronomers to easily distinguish between the two systems in observations, the team concluded.
What’s different about the new study is the type of system the astrophysicists looked at, says Daniel Evans (Harvard-Smithsonian Center for Astrophysics), who in 2010 collaborated on work that suggested retrograde spins, not prograde ones, would cause more powerful jets. “This is a very important paper,” he says, because Tchekhovskoy and McKinney were able to look at a puffed-up disk flooded with magnetic fields, extremes previous simulations have not reached but that may be closer to black holes’ natural environments.
The standard idea of black holes is that nothing comes out. That’s a bit oversimplified. A black hole eating a magnetic field may not actually eat the whole thing, instead leaving part dangling from its mouth. Magnetic fields are like loops — no matter how small the pieces are that you cut a bar magnet into, each piece still has a north and south pole. A black hole can swallow a whole loop, like a Cheerio. But it can also swallow only part of it. In this case the black hole is more like an olive threaded on a loop of spaghetti. The magnetic field line (the spaghetti) “can go down all the way through the pole of the event horizon, pass through the black hole, and come out on the other side,” Tchekhovskoy says. “That’s the beauty of magnetic fields.” It is these field lines threaded through the black hole that funnel the jets out.
For a black hole stuffed with as many field lines as it can swallow from the accreting material, only the disk’s thickness and how fast the black hole spins matter in determining the outflows’ efficiency, the duo discovered. When the researchers looked at various spin magnitudes, they found that prograde spins generated outflows several times more efficient than retrograde spins did.
Tchekhovskoy explains that previous studies have come to different conclusions because they haven’t studied the most powerful jets creatable for each spin. “When you are figuring out which car is fastest, you push the accelerator pedal all the way to the floor,” he says. In this case, the pedal is the amount of magnetic field concentrated in the system. Higher concentration, stronger jet. A prograde setup allows the accretion flow to drag more magnetic field in toward the black hole, but that's just one factor, Tchekhovskoy says. A full understanding of what's afoot remains elusive.
You can watch videos of a prograde black hole and a retrograde black hole on YouTube. For what happens when the black hole swallows too much magnetic field, look at the prograde video, around 1:15. The eruption happens because the magnetic field is so strong (there are so many spaghetti noodles in the black hole’s mouth) that the field squeezes the disk out and prevents material from falling in, undermining the black hole’s gravitational pull.