An X-ray telescope points at a neighboring galaxy, and sees a point that wasn't there yesterday. A month later, it blazes hundreds of times brighter than the Sun, and a year after that, it fades again. Such points are called ultraluminous X-ray sources, and each one is a close pair: an ordinary star and a black hole or neutron star, whirling around each other.
The black hole pulls matter from its companion star. The gas spirals in, heats up to millions of degrees, and shines in X-rays. If the star feeds gas unevenly, in clumps, the brightness jumps. Astronomers studied two galaxies — NGC 4631 and NGC 1097—using images spanning 25 years and found three such variable sources. One of them, in galaxy NGC 4631, would flare up and then nearly disappear: its X-ray flux changed by more than a factor of ten. Based on the pattern of emission, the scientists concluded that a black hole with a mass of several Suns is at work there.
Such pairs are living laboratories. They show how black holes strip matter from stars and what happens to gas on the brink of falling in. Each new flare is like a frame from a movie about the life of a binary system, stretched over millions of years.
Ultraluminous X-ray sources shine so powerfully that if one were placed at the distance of the nearest star, it would be visible from Earth with the naked eye as a bright point in the sky.