The night sky looks still, but sometimes one of the stars suddenly begins to brighten. That's what happened with the system AT 2019wey: it was spotted by the TESS telescope, which usually hunts for planets around other stars, but happened to be imaging that patch of sky every half hour. Thanks to this, astronomers saw for the first time how a black hole in a binary system wakes up — literally frame by frame.
In the AT 2019wey system, the black hole is pulling material from a neighboring ordinary star. The material spirals into a disk, like water down a drain, and gradually falls onto the black hole. Sometimes this flow suddenly intensifies — and the disk flares up. Previously, such flares were caught in full swing, but this time we got to see the very beginning: the brightness grew smoothly, like a light bulb warming up. Over 25 days of observations, the light became hundreds of times brighter.
The scientists pinpointed the exact start of the flare: January 27, 2020, around 8 p.m. Greenwich time. Curiously, a faint X-ray glow appeared two days earlier. Apparently, the inner part of the disk heats up first, and only then does the light spread outward — like a fire that starts in the middle of a campfire, but the flames aren't visible right away.
The astronomers also looked for rapid flickers in brightness — maybe the black hole "blinks" every few hours. But they found nothing of the sort: the light grew steadily, without fluctuations. This narrows down the possible causes of the flare and helps us understand what exactly pushes material into the black hole.
The TESS telescope that caught this flare was launched to find planets around other stars — but it also images the entire sky every half hour, and such accidental discoveries happen regularly.