Black holes are famous for being ... well, black! They are so great in mass that they bend space around them, and nothing can escape, famously not even light itself. So, how are some black holes 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 people are still determining why.
A new paper in Nature Astronomy – focused on this twinkling phenomenon, piggy-backing on NASA's asteroid defense effort to watch over 5,000 of the fastest-growing black holes in the sky for five years – reports an answer: A kind of turbulence driven by friction and intense gravitational and magnetic fields.
The paper studied supermassive black holes, which sit at galaxies' centers and are as massive as millions or billions of Suns.
Our galaxy, the Milky Way, has one of these giants at its center, with a mass of about four million Suns. But, mostly the 200 billion stars that make up the rest of the galaxy – including our Sun – happily orbit around the black hole at the center.
However, things are only so peaceful in some 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.
This 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.
In the new study, scientists looked closely at the feeding process among the 5,000 fastest-growing black holes in the Universe.
In earlier studies, they discovered the black holes with the most voracious appetite. Last year, they found a black hole that eats an Earth's-worth of stuff every second. In 2018, they found one that eats a whole Sun every 48 hours.
In some black holes, the material on its way into the hole spirals into a glowing "accretion disc" 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 – too far for us to see any disc detail. We have some images of accretion discs around nearby black holes, but they are merely breathing in some cosmic gas rather than feasting on stars.
In the new work, scientists used data from NASA's ATLAS telescope in Hawaii. It scans the entire sky every night – weather permitting – monitoring for asteroids approaching Earth from the outer darkness.
The twinkling of these black holes can tell us something about accretion discs.
Scientists thought magnetic fields could be responsible for the twinkle. But would the answer prove this simple with no further complexities?
A separate question is whether "simple" is the right word for turbulence in an ultra-dense, out-of-control environment embedded in intense gravitational and magnetic fields where space is bent to a breaking point. However, using statistical methods, scientists measured how much the light emitted from our 5,000 discs flickered over time. The pattern of dashing in each one looked different.
But they began to see intriguing patterns by sorting them by size, brightness, and color. They were able to determine the orbital speed of each disc – and once they set their clock to run at the disc's pace, all the flickering patterns started to look the same.
This universal behavior is indeed predicted by the theory of “magneto-rotational instabilities" proposed by astrophysicists Steven Balbus and John Hawley in 1998, where they described how magnetic fields could cause turbulence in the discs.
So, it means these mind-boggling maelstroms are "simple," after all.
