The light of a distant lighthouse does not go out all at once: first it flares brightly, then slowly fades into darkness. That is how the X-ray pulsar 4U 0115+63 behaves — a neutron star that periodically brightens when gas from a companion ordinary star falls onto it. A neutron star is the remnant of a massive star that exploded, compressed to the size of a city, with a magnetic field billions of times stronger than Earth's. Such a pulsar's outbursts last weeks or months, and then it quiets down.

Using the NICER space telescope, a team of astronomers has for the first time traced the entire fading of 4U 0115+63—from the bright phase to complete quiescence. Previously, it was thought that the decay was explained by the "propeller effect": the star's magnetic field spins up the gas and flings it away, like fan blades. But the new data show something different. The gas disk around the star simply cools and stops flowing inward, like cooled tea stops emitting steam. The star, meanwhile, continues to shine for a long time, slowly releasing accumulated heat.

This picture is well described by the thermal instability model of the disk — the same one that explains outbursts in ordinary, non-magnetic binary systems. The scientists applied this model to a whole set of X-ray pulsars with different parameters and found agreement with observations. This means that the propeller effect is not needed as the main mechanism for the fading. Instead, the very fact of fading allows estimating the strength of the star's magnetic field — by how quickly it fades.

Thus, a distant lighthouse, extinguished after an outburst, revealed its hidden properties. Its light, fading into darkness, left behind not only an image but also a key to the magnetic force that otherwise cannot be measured.