Two new papers probe the matter that's right on the edge of falling in.
Ars Technica said:While black holes themselves swallow any light beyond their event horizon, the area outside the event horizon tends to emit lots of light. That's because the material falling in toward the black hole is extremely energetic as it sheds angular momentum and crashes in to other material in orbit around the black hole. So, while we can't image a black hole directly, we can infer some things about its properties using light from the environment it creates.
This week saw the publication of two papers that edge in to the area close to the event horizon, imaging events in an area that includes some of the closest stable orbits to the black hole. And, in doing so, one of them finds that a supermassive black hole is spinning so fast that a location on its surface would move at roughly half the speed of light.
Both of these papers take advantage of periodic outbursts that happen when the black hole starts to feed on new material. That material heads into the hole via a flat structure centered on the black hole called an accretion disk. Its arrival heats the disk up, causes the black hole to brighten, and causes changes in the local environment. The questions that these two papers focus on is what these changes can tell us about the black hole and the environment nearby.
One of the studies focuses on a stellar-mass black hole, or one that's typically less than 10 times the mass of the Sun. In response to some infalling matter, one of these black holes created a transient event called MAXI J1820+070, which gets part of its name from the International Space Station's Monitor of All-sky X-ray Image instrument, or MAXI. The event's discovery was then followed up by observations using a different piece of ISS-based hardware, the Neutron star Interior Composition Explorer (or NICER). NICER has the ability to perform very rapid measurements of the X-rays coming from astronomical sources, making it great for tracking short-term changes in an object.
In this case, NICER was used to perform what's called "reverberation analysis." This technique relies on the fact that, in addition to the accretion disk, black holes have a corona, which is a blob of energetic material above and below the plane of the disk. This corona will produce X-rays of its own, which instruments can detect. But those X-rays also run into the accretion disk, and some of them get reflected back toward us. These light reverberations can tell us something about the details of the accretion disk.
In this case, they solved a bit of a mystery. In the supermassive black holes at the center of galaxies, imaging had suggested that the accretion disk extended in to the closest stable orbit possible around the black hole. But measurements of stellar-mass black holes indicated that the edge of the accretion disk was much further out. Since there were no obvious reasons why the physics should change with size, these measurements were a little confusing.
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