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at the end of 2017 a galaxy 236 million

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light years away began a rare and

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dramatic transformation

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it's an event astronomers are still

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puzzling over

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one set off by changes near its central

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black hole

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they first explained this as a tidal

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disruption event

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that's when a star wanders so close to a

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supermassive black hole that it's torn

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apart

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a new study of observations spanning the

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entire event suggests a different cause

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the trigger may have been a flip in the

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magnetic field in the disk of material

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around the black hole

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the sun's magnetic field reverses

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polarity every 11 years

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on longer time scales even earth's

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magnetic field flips

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starting in december 2017 the galaxy

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began to brighten invisible and

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ultraviolet light

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the source of this brightening appeared

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to be the disc of material around the

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supermassive black hole at the galaxy's

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center

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[Music]

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it peaked three months later at nearly

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that's when regular monitoring with

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nasa's swift satellite began

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as the visible and uv brightened x-rays

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from the galaxy dimmed

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by august 2018 the higher energy x-rays

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had disappeared completely

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a few months later the high energy

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x-rays came back even brighter than

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before

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they returned to normal within a year

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these x-rays come from a cloud of

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super-hot particles near the black hole

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it's a feature called the corona which

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is formed by the strong magnetic field

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the lack of higher energy x-rays means

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that this structure was essentially gone

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based on observations from swift

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europe's xmm satellite and ground-based

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optical and radio telescopes here's what

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the visible and uv flare results when

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the flow of matter into the black hole

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increases

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this may have started when the magnetic

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field in the disk's outer regions began

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to flip

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the weakened magnetic field can no

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longer support the corona which vanishes

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the flipped magnetic field gains

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strength restoring the x-ray corona but

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the inward flow of matter is still high

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so this emission is stronger than it was

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before the flare

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finally the corona and disc returned to

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their states before the flare

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now with a flipped magnetic field

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rapid changes in uv and visible light

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have only been observed in a few dozen

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active galaxies like this one

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but this is the first time x-rays have

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been seen to drop out as other

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wavelengths brighten these surprising

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events offer a tantalizing glimpse at

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the extreme forces at work near an