Location and energy of electrons producing the radio bursts from AD Leo observed by FAST in December 2021

Abstract

Context. In a recent paper, we presented circularly polarised radio bursts detected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) from the flare star AD Leo over 2-3 December 2021. These bursts have been attributed to the electron cyclotron maser (ECM) instability. Aims. In that context, we have adopted two independent and complementary approaches, inspired by the study of auroral radio emissions from Solar System planets. Our goal is to constrain, for the first time, the source location (magnetic shell, height) and the energy of the emitting electrons. Methods. These two approaches consist of (i) modelling the overall occurrence of the emission with the ExPRES code and (ii) fitting the drift rate of the fine structures observed by FAST. Results. We obtained consistent results, pointing at 20-30 keV electrons on magnetic shells with an apex at 2-10 stellar radii. The emission polarisation observed by FAST and the magnetic topology of AD Leo appear to favour X-mode emission from the southern magnetic hemisphere, allowing us to set constraints on the plasma density scale height in the star's atmosphere. Conclusions. We demonstrate that sensitive radio observations with high time-frequency resolutions, coupled with modelling tools such as ExPRES, along with analytical calculations and stellar magnetic field measurements, allow us to remotely probe stellar radio environments. We provide elements of comparison with Solar System radio bursts (Jovian and Solar), establish hypotheses about the driver of AD Leo's radio bursts, and discuss the perspectives of future observations, particularly at very low frequencies (<100 MHz).