Broadband 12-26 GHz Radio Radiation Reveals Evidence for Micro-flares on AU Mic

We present sequential 12-18 and 18-26 GHz radio-band (Ku, K) VLA observations of the 22 Myr dM1e star AU Mic. We detect two flares and two marginal events over a total of 3 contiguous hours on source, resulting in a radio flare rate of about 1 flare hour-1. While this rate is consistent with previous Ku-band observations, both flaring (< 1 mJy) and quiescent (about 0.4 mJy) flux densities are significantly lower. Furthermore, the quiescent spectral shape here is distinct, allowing for unique constraints on the radio-emitting sources of AU Mic. The time-averaged quiescent spectrum is best described by gyrosynchrotron radiation with a peak around 17 GHz and an optically thin spectral index of α about-0.6. We estimate that the source regions have magnetic field strengths of about 1 kG and cover a fraction of < 0.5% of the stellar surface, yet the instantaneous total electron kinetic energies are about 10 28 erg. The power-law index describing the distribution of electrons with energy derived from the spectral index, δ about 2, implies a near-continuous injection of electrons. This could arise from micro-flares that occur over the surface of AU Mic that sustain the radio radiation. One clear flare per band occurs, with decay-to-rise e-folding time ratios of 3 - 4, indicating magnetic trapping of the electrons. The Ku-band flare is optically thick during the rise and peak times, indicating a peak frequency above 18 GHz. Together, these quiescent and flaring characteristics suggest that continuous, unresolved micro-flaring and magnetic trapping dominate the non-thermal radio emission of active M-dwarf coronae.