Starspots as the origin of ultrafast drifting radio bursts from an active M dwarf

Detecting coherent radio bursts from nearby M dwarfs provides opportunities for exploring their magnetic activity and interaction with orbiting exoplanets. However, it remains uncertain whether the emission is related to flare-like activity similar to the Sun or magnetospheric process akin to magnetized planets. Using observations (1.0 to 1.5 gigahertz) taken by the Five-hundred-meter Aperture Spherical radio Telescope, we found a type of millisecond-scale radio bursts with exceptionally high-frequency drift rates (~8 gigahertz per second) from an active M dwarf, AD Leo. The ultrafast drift rates point to a source region with a notably low magnetic scale height (<0.15 r⋆ , r⋆ as the stellar radius), a feature not expected in a commonly assumed dipole-like global field but highly possible in localized strong-field structures, i.e., starspots. Our findings suggest that a concentrated magnetic field above starspots could be responsible for some of the most intense radio bursts from M dwarfs, supporting a solar-like electron acceleration mechanism.