“I’ll never, ever be full. I’ll always be hungry. Obviously, I’m not talking about food.” -Dwayne ‘The Rock’ Johnson
Only a very small percentage of galaxies have active supermassive black holes. While the black holes themselves are common, they only rarely feed, gaining a huge influx of matter to accelerate and send jets and other emission out. When a galaxy does become active, they can appear in any number of interesting manifestations, dependent on their orientation relative to us.
But most galaxies, on human timescales, will either appear permanently off or on. A galaxy can turn on from a merger, accretion or just a chance encounter with a star cluster or gas cloud, but for the first time, we’ve seen one turn on and then off in great detail. As it turns out, the X-ray emissions are key to watching the feeding stop, and seeing the galaxy fade as a result.