Why do black holes twinkle?

Black holes are bizarre things, even by the standards of astronomers. Their mass is so great, it bends space around them so tightly that nothing can escape, even light itself.

And yet, despite their famous blackness, some black holes are quite visible. The gas and stars these galactic vacuums devour are sucked into a glowing disk before their one-way trip into the hole, and these disks can shine more brightly than entire galaxies.

Stranger still, these black holes twinkle. The brightness of the glowing disks can fluctuate from day to day, and nobody is entirely sure why.

We piggy-backed on NASA’s asteroid defense effort to watch more than 5,000 of the fastest-growing black holes in the sky for five years, in an attempt to understand why this twinkling occurs. In a new paper in Nature Astronomy, we report our answer: a kind of turbulence driven by friction and intense gravitational and magnetic fields.

Gigantic star-eaters

We study supermassive black holes, the kind that sit at the centers of galaxies and are as massive as millions or billions of Suns.

Our own galaxy, the Milky Way, has one of these giants at its center, with a mass of about four million Suns. For the most part, the 200 billion or so stars that make up the rest of the galaxy (including our Sun) happily orbit around the black hole at the center of the Milky Way.

However, things are not so peaceful in all galaxies. When pairs of galaxies pull on each other via gravity, many stars may end up tugged too close to their galaxy’s black hole. This ends badly for the stars: they are torn apart and devoured.

We are confident this must have happened in galaxies with black holes that weigh as much as a billion Suns, because we can’t imagine how else they could have grown so large. It may also have happened in the Milky Way in the past.

Black holes can also feed in a slower, more gentle way: by sucking in clouds of gas blown out by geriatric stars known as red giants.

Feeding time

In our new study, we looked closely at the feeding process among the 5,000 fastest-growing black holes in the universe.

In earlier studies, we discovered the black holes with the most voracious appetite. Last year, we found a black hole that eats an Earth’s-worth of stuff every second. In 2018, we found one that eats a whole Sun every 48 hours.

But we have lots of questions about their actual feeding behavior. We know material on its way into the hole spirals into a glowing “accretion disk” that can be bright enough to outshine entire galaxies. These visibly feeding black holes are called quasars.

Most of these black holes are a long, long way away — much too far for us to see any detail of the disk. We have some images of accretion disks around nearby black holes, but they are merely breathing in some cosmic gas rather than feasting on stars.