A stone in water creates ripples; a moving stone pushes the ripples forward, crowding them. This effect, discovered by Doppler, also holds for light: when a source moves, its color shifts. Astronomers used it to peer into a binary system where a neutron star — a superdense remnant, sometimes acting as a pulsar — pulls matter from a companion star. The gas spirals into a disk, heated to X-ray glow.

Using spectroscopy — breaking light into its colors — scientists measured the shift of an iron line and built a velocity map. They expected X-rays from the disk or corona, but the main source turned out to be a stream spilling over the disk's edge, like water from an overflowing cup. The same 'waterfall' is seen in optical light.

This new method, X-ray Doppler tomography, lets us directly see the movement of gas in such systems — a true ultrasound of cosmic currents.