What makes supermassive black holes like Sagittarius A* shine?

Black holes are, by definition, places where gravity is so strong that not even light can escape. So they are black – dark – and impossible to imagine. To the right?

As evidenced by the success of the Event Horizon telescope, we can pictures of black holes. But what these pictures show is shadow of the black hole on a brighter background. The light you see does not come from the black hole itself, but from a region around it called the accretion disk. It is a disc-like structure of matter falling inward, pulled to its doom by the gravity of the black hole. Once it crosses the event horizon, or the point of no return, it disappears from view. But outside the event horizon, we can still see it – and any light it gives off.

Build an accretion disk

Why is there a disk? Can’t everything that falls fall into the black hole at the same time? It can’t! Because everything in the universe has its own moment (in particular, angular momentum), it cannot fall directly into the gaping maw of a black hole. Instead, it has to lose that momentum first, and it does so by swirling around in a disk, getting closer and closer until it finally crosses the event horizon and disappears.

The physics behind this is complex, but the simplified story is that accretion disks transfer angular momentum from their inner regions to their outer regions through processes such as turbulence and friction. Essentially, as matter swirls closer to the black hole, it collides and rubs against After material already on disk. This slows the fall of materials and generates heat. And when it heats up (here, we take tens of millions of degrees), it shines.

And astronomers are real pros at looking for things that sparkle.

We see light from accretion disks over a wide swath of the electromagnetic spectrum. X-rays or visible light usually come from the hot disc itself. Also, the light coming directly from the accretion disk is not constant. It changes, flickering on timescales that can range from minutes and hours for small stellar-mass black holes to weeks or months for supermassive black holes that sit at the centers of galaxies. This flickering is random, and the researchers suspect it may be due to processes within the disk, such as regions of higher turbulence or even clumps of matter approaching or falling into the black hole.

The rate of variability – although still random – scales with the mass of the black hole. More massive black holes with larger accretion disks change more slowly than those with less mass and smaller accretion disks. The Event Horizon Telescope (EHT) has detected changes in accretion disks around supermassive black holes in M87 (the first black hole ever photographed, released in April 2019) and 3C 279 “over the course of a week”, Dan Marrone at the University of Arizona, a member of the EHT scientific council and coordinator of the data analysis working group, said Astronomy in 2019. “And since Sgr A* is 1,500 times less massive than M87, it varies all the more quickly. Reduce a week by 1,500 times, you’re talking minutes.

Taking a clear photo is extremely difficult when the subject you are trying to photograph literally changes from minute to minute, while you try to capture it. So when it came to getting an image of Sgr A*, Marrone said, it wasn’t necessarily about getting more data. Instead, “figuring out how to deal with a source that changes while we’re watching it is a bigger deal.”

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