At night in the forest, you only see fireflies where your flashlight shines. That's how ultraluminous X-ray sources were hunted before: bright points in other galaxies that outshine an ordinary stellar-mass black hole in X-rays. Telescopes stared at galaxies chosen for other tasks, missing most of these fireflies.

Now the eROSITA telescope has uniformly scanned the entire western sky, as if we'd turned on lights all across the forest. Astronomers found 90 reliable candidates, 53 of which no one had ever seen before. Each source was checked by hand to avoid confusion with distant quasars.

Comparison with old lists showed that many known sources have faded while new ones have flared up. These objects are fickle: they brighten and dim over months. So previous catalogs were not just incomplete, they were skewed.

Now we have an honest list of all the fireflies. With it, we can study which galaxies they live in and whether any of them are intermediate-mass black holes, which have yet to be found.