A Strange Black Hole Is Shooting Out Wobbly Jets Because It’s Dragging Spacetime
Some 7,800 light-years away, in the constellation of Cygnus, lies a most peculiar black hole. It’s called V404 Cygni, and in 2015, telescopes around the world stared in wonder as it woke from dormancy to devour material from a star over the course of a week.
That one event provided such a wealth of information that astronomers are still analysing it. And they have just discovered an amazing occurrence: relativistic jets wobbling so fast their change in direction can be seen in mere minutes.
And, as they do so, they puff out high-speed clouds of plasma.
“This is one of the most extraordinary black hole systems I’ve ever come across,” said astrophysicist James Miller-Jones of the International Centre for Radio Astronomy Research (ICRAR) at Curtin University in Australia back in April.
V404 Cygni is a binary microquasar system consisting of a black hole about nine times the mass of the Sun and a companion star, an early red giant slightly smaller than the Sun.
The black hole is slowly devouring the red giant; the material siphoned away from the star is orbiting the black hole in the form of an accretion disc, a bit like water circling a drain. The closest regions of the disc are incredibly dense and hot, and extremely radiant; and, as the black hole feeds, it shoots out powerful jets of plasma, presumably from its poles.
Scientists don’t know the precise mechanism behind jet production. They think material from the innermost rim of the accretion disc is funnelled along the black hole’s magnetic field lines, which act as a synchrotron to accelerate the particles before launching them at tremendous velocities.
But V404 Cygni’s wobbly jets, shooting out in different directions at different times, on such rapidly changing timescales, and at velocities up to 60 percent of the speed of light, are in a class of their own.
“We think the disc of material and the black hole are misaligned,” Miller-Jones said. “This appears to be causing the inner part of the disc to wobble like a spinning top and fire jets out in different directions as it changes orientation.”
It’s a bit like a spinning top that starts to wobble as it’s slowing down, the researchers said. This change in the rotational axis of a spinning body is called precession. In this particular instance, we have a handy explanation for it courtesy of Albert Einstein.
In his theory of general relativity, Einstein predicted an effect called frame-dragging. As it spins, a rotating black hole’s gravitational field is so intense that it essentially drags spacetime with it. (This is one of the effects scientists hoped to observe when they took a picture of Pōwehi.)
In the case of V404 Cygni, the accretion disc is about 10 million kilometres (6.2 million miles) across. The misalignment of the black hole’s rotational axis with the accretion disc has warped the inner few thousand kilometres of said disc.
The frame-dragging effect then pulls the warped part of the disc along with the black hole’s rotation, which sends the jet careening off in all directions. In addition, that inner section of the accretion disc is puffed up like a solid doughnut that also precesses.
“This is the only mechanism we can think of that can explain the rapid precession we see in V404 Cygni,” Miller-Jones said.
It’s so fast that the usual method radio telescopes use for imaging space were practically useless. Usually, these devices rely on long exposures, observing a region for several hours at a time, moving across the sky to track their target. But in this case, the method produced images too blurred to be of use.
So the team had to use a different method, taking 103 separate images with exposure times of just 70 seconds and stitching them together to create a movie – and sure enough, there were the wibbly wobbly spacetimey jets.
“We were gobsmacked by what we saw in this system – it was completely unexpected,” said physicist Greg Sivakoff of the University of Alberta.
“Finding this astronomical first has deepened our understanding of how black holes and galaxy formation can work. It tells us a little more about that big question: ‘How did we get here?'”
The research was published in Nature.
A version of this article was first published in April 2019.