A 7 day Multiwavelength Flare Campaign on AU Mic. II. Electron Densities and Kinetic Energies from High-frequency Radio Flares

Abstract

M dwarfs are the most common type of star in the solar neighborhood, and many exhibit frequent and highly energetic flares. To better understand these events across the electromagnetic spectrum, a campaign observed AU Mic (dM1e) over 7 days from the X-ray to radio regimes. Here, we present high-time-resolution light curves from the Karl G. Jansky Very Large Array (VLA) Ku band (12–18 GHz) and the Australia Telescope Compact Array (ATCA) K band (16–25 GHz), which observe gyrosynchrotron radiation and directly probe the action of accelerated electrons within flaring loops. Observations reveal 16 VLA and three ATCA flares of varying shapes and sizes, from a short (30 s) spiky burst to a long-duration (∼5 hr) decaying exponential. The Ku-band spectral index is found to often evolve during flares. Both rising and falling spectra are observed in the Ku band, indicating optically thick and thin flares, respectively. Estimations from optically thick radiation indicate higher loop-top magnetic field strengths (∼1 kG) and sustained electron densities (∼106 cm−3) than previous observations of large M dwarf flares. We estimate the total kinetic energies of gyrating electrons in optically thin flares to be between 1032 and 1034 erg when the local magnetic field strength is between 500 and 700 G. These energies are able to explain the combined radiated energies from multiwavelength observations. Overall, values are more aligned with modern radiative-hydrodynamic simulations of M dwarf flares, and future modeling efforts will better constrain findings.