Andrea Santangelo

Two telescopes — Swift/BAT and NuSTAR — have been watching the same pair of stars for twenty years straight: a neutron star and its giant companion.

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.

In 2026, a black hole in the binary system GS 1354-64 woke up: it began pulling matter from its companion star, and a scorching disk swirled around it.

The black hole GRS 1915+105 usually hides behind a dense gas curtain: its X-ray emission is heavily absorbed on its way to us. But in April 2023, it suddenly flared: the X-ray flux jumped nearly tenfold.

A research probe is planned for the nearest black hole — a region of spacetime whose gravity traps even light. After a hundred years of flight, the spacecraft will enter orbit around the mysterious object to answer a key question: are we looking at a black hole or a neutron star — the ultra-dense remnant of a massive star, compressed to nuclear…