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 disc before their one-way trip into the hole, and these discs can shine more brightly than entire galaxies.
Stranger still, these black holes twinkle. The brightness of the glowing discs 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.
Why do black holes twinkle? We studied 5,000 star-eating behemoths to find out