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

TL;DR

This study combines observational data and modeling to locate the source and estimate the energy of electrons causing radio bursts from AD Leo, revealing insights into stellar magnetic environments and emission mechanisms.

Contribution

It introduces two independent approaches, modeling and drift-rate fitting, to constrain electron energy and source location in stellar radio emissions for the first time.

Findings

Electrons with 20-30 keV energy are responsible for the bursts.

Radio emissions originate from magnetic shells at 2-10 stellar radii.

X-mode emission from the southern magnetic hemisphere is favored.

Abstract

In a recent paper, we presented circularly polarized radio bursts detected by the radio telescope FAST from the flare star AD Leo on December 2-3, 2021, which were attributed to the electron cyclotron maser instability. In that context we use here two independent and complementary approaches\pz{, inspired from the study of auroral radio emissions from solar system planets,} to constrain for the first time the source location (magnetic shell, height) and the energy of the emitting electrons. These two approaches consist of (i) modeling the overall occurrence of the emission with the ExPRES code, and (ii) fitting the drift-rate of the fine structures observed by FAST. We obtain consistent results pointing at 20-30 keV electrons on magnetic shells with apex at 2-10 stellar radii. Emission polarization observed by FAST and magnetic topology of AD Leo favour X-mode emission from the southern magnetic hemisphere, from which we draw constraints on the plasma density scale height in the star's atmosphere. We demonstrate that sensitive radio observations with high time-frequency resolutions, coupled to modelling tools such as ExPRES, analytical calculations and stellar magnetic field measurements, now allow us to remotely probe stellar radio environments.} We provide elements of comparison with solar system radio bursts (Jovian and Solar), emit hypotheses about the driver of AD Leo's radio bursts and discuss the perspectives of future observations, in particular at very low frequencies (<100 MHz).