A waterfall that doesn't heat up from the fall but freezes — that's roughly how gas behaves as it flows onto a neutron star in an X-ray pulsar when the star shines dimly. Usually, infalling matter is heated to a white-hot glow, but new work shows the opposite.
The secret lies in the interplay of three forces. X-rays from the star heat the gas — much like a spoon heats up in hot tea. Compression during the fall also adds heat, similar to air in a bicycle pump. But the main role is played by cooling: the gas loses energy through radiation. This happens mainly through free-free transitions (when an electron flies past a proton and emits light) and cyclotron radiation in a strong magnetic field. At low luminosity, cooling wins, and the flow cools down.
A surprising consequence: when the temperature drops, the plasma is no longer fully ionized. Electrons and protons partially recombine into neutral hydrogen. External radiation ionizes only a thin layer near the surface. This means that at low luminosity, accretion proceeds through a partially ionized medium, not through fully ionized plasma as previously thought.
At temperatures of tens of electron-volts, the gas is cooler than the surface of many stars: 1 eV corresponds to about 11,600 degrees Celsius, but an ordinary star can be heated to millions of degrees.