A lighthouse sends its beam through a storm of its own mirrors: that's roughly how fast radio bursts behave when trying to leave a magnetar's vicinity. A magnetar is a neutron star with a magnetic field a quadrillion times stronger than Earth's. In its inner magnetosphere, gigahertz radio waves transform into special magnetosonic waves and immediately enter a triple resonance. Energy is pumped into closed Alfvén oscillations — the trembling of magnetic field lines.

Using three-dimensional simulations, the authors showed that the radio signal rapidly decays as it excites these oscillations. In a quiet dipole field, damping is effective within 10–100 stellar radii. Beyond that, Alfvén waves suffer 'charge starvation'—they lack particles to grow, and the process halts. But if the burst occurs during an eruption, when the star ejects a relativistic magnetic outflow, the resonance works at greater distances. The signal may only fade at 100–1000 radii from the surface. This sets a hard limit: the most powerful bursts must die out before reaching outer space.

Thus, the magnetic storm around the star acts as a natural filter. Only those radio cries that manage to slip through narrow gaps or are carried out with the plasma flow reach our telescopes. This explains why fast radio bursts are so rare and unpredictable.