Twinkle, twinkle, big black hole
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The black hole at the center of our Milky Way galaxy is twinkling.
Well, not exactly. The black hole itself doesn’t emit any light, so it can’t twinkle. But due to its huge gravity, the black hole attracts gas from its immediate surroundings. Before the gas plunges into the abyss, it forms a huge rotating disk that swirls around the black hole. The gas in the disk heats up and starts to emit light and other forms of radiation. ALMA has now seen slow flickerings in the millimeter waves from this ‘accretion disk’. Millimeter waves are the short-wavelength radio waves that ALMA is sensitive to.
Earlier observations already revealed flares from the Milky Way’s central black hole at infrared and X- ray wavelengths. The variations at millimeter wavelengths are much smaller, and have never been seen before. The flickering is not random, but it isn’t strictly periodic, either. On average, the black hole is temporarily brightening every half hour or so.
Astronomers know that the inner edge of the hot accretion disk is orbiting the black hole once every half hour, with a tremendously high velocity – about one-third of the speed of light. Therefore, they suspect that the twinkling is related to the rotation of the disk.
Suppose there’s a small, extremely hot pocket of gas in the disk, maybe because of some turbulence. As the hot region orbits the black hole once every half hour, it will alternately move towards us and away from us. Because of the high velocity, the radiation from the hot region is strongly boosted as it moves in our direction. That would explain the quasi-periodic flickering.
The ALMA observations provide astronomers with information of what happens extremely close to the black hole: a mere 30 million kilometers or so. ALMA has also taken part in an effort to take an actual photograph of the black hole’s immediate surroundings, by teaming up with other millimeter- wave telescopes around the world. However, the rapid flickering makes it hard to combine the various measurements into one single image.
What?
The supermassive black hole in the core of our Milky Way galaxy is known as Sagittarius A* (pronounced as Sagittarius A-star). It weighs in at some 4 million times the mass of the Sun. Still, there’s nothing to be afraid about, as it is 27,000 light-years away – much too far to do any harm.
Sagittarius A* is surrounded by a large accretion disk of hot gas. Eventually, the gas in this disk will end up in the black hole, but new gas is continuously streaming into the disk from the outside. The inner edge of the disk lies about 30 million kilometers from the black hole’s edge. That’s just one-fifth of the distance between the Sun and the Earth.
Who?
The ALMA observations of Sagittarius A* were carried out by a team of Japanese astronomers, led by Yuhei Iwata. Yuhei is a graduate student at Keio University at Yokohama, Japan. He worked together with his colleagues Tomoharu Oka, Masato Tsuboi, Makoto Miyoshi, and Shunya Takekawa. They used ALMA to study the Milky Way’s central black hole for over an hour per day, on ten days between October 5 and October 20, 2017. The team has published the results of their observations and analysis in The Astrophysical Journal Letters in April 1, 2020.
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