Science & Nature

Why Do Black Holes Twinkle?

Why Do Black Holes Twinkle? thumbnail
Black hole devouring star
In this illustration, light from a smaller black hole (left) curves around a larger black hole and forms an almost-mirror image on the other side. The gravity of a black hole can warp the fabric of space itself, such that light passing close to the black hole will follow a curved path around it.

In our latest work, we 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.

These whole-sky scans also happen to provide a nightly record of the glow of hungry black holes deep in the background. Our team put together a five-year movie of each of those black holes, showing the day-to-day changes in brightness caused by the bubbling and boiling glowing maelstrom of the accretion disc.

The twinkling of these black holes can tell us something about accretion discs.

In 1998, astrophysicists Steven Balbus and John Hawley proposed a theory of “magneto-rotational instabilities” that describes how magnetic fields can cause turbulence in the discs. If that is the right idea, then the discs should sizzle in regular patterns. They would twinkle in random patterns that unfold as the discs orbit. Larger discs orbit more slowly with a slow twinkle, while tighter and faster orbits in smaller discs twinkle more rapidly.

But would the discs in the real world prove this simple, without any further complexities? (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 itself is bent to breaking point is perhaps a separate question.)

Using statistical methods we measured how much the light emitted from our 5,000 discs flickered over time. The pattern of flickering in each one looked somewhat different.

But when we sorted them by size, brightness and color, we began to see intriguing patterns. We were able to determine the orbital speed of each disc — and once you set your clock to run at the disc’s speed, all the flickering patterns started to look the same.

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This universal behavior is indeed predicted by the theory of “magneto-rotational instabilities.” That was comforting. It means these mind-boggling maelstroms are “simple” after all.

And it opens new possibilities. We think the remaining subtle differences between accretion discs occur because we are looking at them from different orientations.

The next step is to examine these subtle differences more closely and see whether they hold clues to discern a black hole’s orientation. Eventually, our future measurements of black holes could be even more accurate.

Christian Wolf is an associate professor of astronomy and astrophysics at Australian National University. He receives funding from the Australian Research Council (ARC) and is a member of the Astronomical Society of Australia (ASA).

This article is republished from The Conversation under a Creative Commons license. You can find the original article here.

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