WEBVTT FILE

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[Music throughout] At the end of 2017,

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a galaxy 236 million light-years away began

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a rare and dramatic transformation. It’s an event

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astronomers are still puzzling over, one set off by changes near

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its central black hole. They first explained this

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as a tidal disruption event. That’s when a star wanders so close

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to a supermassive black hole that it’s torn apart.

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A new study of observations spanning the entire event

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suggests a different cause. The trigger may have been

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a flip in the magnetic field in the disk of material around the

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black hole. The Sun’s magnetic field reverses polarity

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about every 11 years. On longer timescales, even Earth’s magnetic

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field flips. Starting in December 2017,

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the galaxy began to brighten in visible and ultraviolet light.

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The source of this brightening appeared to be the disk of material around

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the supermassive black hole at the galaxy’s center.

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It peaked three months later at nearly 100 times

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its previous brightness.

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That’s when regular monitoring with NASA’s Swift satellite began.

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As the visible and UV brightened, X-rays from the galaxy dimmed.

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By August 2018, the higher-energy X-rays had

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disappeared completely. A few months later,

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the high-energy X-rays came back, even brighter than before.

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They returned to normal within a year.

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These X-rays come from a cloud of superhot particles near the black hole.

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It’s a feature called the corona, which is formed by the strong magnetic field.

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The lack of higher-energy X-rays means that this structure

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was essentially gone. Based on observations from Swift,

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Europe’s XMM satellite, and ground-based optical and radio telescopes,

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here’s what may have happened.

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The visible and UV flare results when the flow of matter into the black

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hole increases. This may have started when the magnetic field in

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the disk’s outer regions began to flip.

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The weakened magnetic field can no longer support the corona, which vanishes.

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The flipped magnetic field gains strength, restoring

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the X-ray corona, but the inward flow of matter is still

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high, so this emission is stronger than it was before the flare.

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Finally, the corona and disk return to their states before

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the flare, now with a flipped magnetic field.

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Rapid changes in UV and visible light have only

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been observed in a few dozen active galaxies like this one.

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But this is the first time X-rays have been seen to drop out

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as other wavelengths brighten. These surprising events

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offer a tantalizing glimpse at the extreme forces at work

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near an actively feeding supermassive black hole.

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[Music]

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Telescope images courtesy of: Associated Universities Inc., ASTRON, Gran Telescopio Canarias, Italian Institute of Astrophysics

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NASA