Transcripts of 14148_ChangingLook_AGN_1080

[Music throughout] At the end of 2017, a galaxy 236 million light-years away began a rare and dramatic transformation. It’s an event astronomers are still puzzling over, one set off by changes near its central black hole. They first explained this as a tidal disruption event. That’s when a star wanders so close to a supermassive black hole that it’s torn apart. A new study of observations spanning the entire event suggests a different cause. The trigger may have been a flip in the magnetic field in the disk of material around the black hole. The Sun’s magnetic field reverses polarity about every 11 years. On longer timescales, even Earth’s magnetic field flips. Starting in December 2017, the galaxy began to brighten in visible and ultraviolet light. The source of this brightening appeared to be the disk of material around the supermassive black hole at the galaxy’s center. It peaked three months later at nearly 100 times its previous brightness. That’s when regular monitoring with NASA’s Swift satellite began. As the visible and UV brightened, X-rays from the galaxy dimmed. By August 2018, the higher-energy X-rays had disappeared completely. A few months later, the high-energy X-rays came back, even brighter than before. They returned to normal within a year. These X-rays come from a cloud of superhot particles near the black hole. It’s a feature called the corona, which is formed by the strong magnetic field. The lack of higher-energy X-rays means that this structure was essentially gone. Based on observations from Swift, Europe’s XMM satellite, and ground-based optical and radio telescopes, here’s what may have happened. The visible and UV flare results when the flow of matter into the black hole increases. This may have started when the magnetic field in the disk’s outer regions began to flip. The weakened magnetic field can no longer support the corona, which vanishes. The flipped magnetic field gains strength, restoring the X-ray corona, but the inward flow of matter is still high, so this emission is stronger than it was before the flare. Finally, the corona and disk return to their states before the flare, now with a flipped magnetic field. Rapid changes in UV and visible light have only been observed in a few dozen active galaxies like this one. But this is the first time X-rays have been seen to drop out as other wavelengths brighten. These surprising events offer a tantalizing glimpse at the extreme forces at work near an actively feeding supermassive black hole. [Music] Telescope images courtesy of: Associated Universities Inc., ASTRON, Gran Telescopio Canarias, Italian Institute of Astrophysics NASA