Broadband 12–26 GHz Radio Radiation Reveals Evidence for Microflares on AU Mic
Journal Article
Overview
abstract
Abstract; ; 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 ∼1 hr; −1; . While this rate is consistent with previous; Ku; -band observations, both flaring (<1 mJy) and quiescent (∼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; ≈ −0.6. We estimate that the source regions have magnetic field strengths of ∼1 kG and cover a fraction of <0.5% of the stellar surface, yet the instantaneous total electron kinetic energies are ∼10; 28; erg. The power-law index describing the distribution of electrons with energy derived from the spectral index,; δ; ≈ 2, implies a near-continuous injection of electrons. This could arise from microflares 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 microflaring and magnetic trapping dominate the nonthermal radio emission of active M-dwarf coronae.;
