A 7 Day Multiwavelength Flare Campaign on AU Mic. IV. Quiescent Gyrosynchrotron and Gyroresonance Radiation from 12 to 25 GHz

Abstract We present an analysis of the radio quiescent data from a multiwavelength campaign of the active M dwarf flare star AU Mic (dM1e) that occurred in 2018 October. Using Ku -band data (12–18 GHz) from the Karl G. Jansky Very Large Array and K -band data (17–25 GHz) from the Australia Telescope Compact Array, we find that the quiescent spectrum can be decomposed into two components: one falling with frequency and one that remains flat. The flat component has a relatively steady flux density of 0.64 ± 0.14 mJy. The falling component varies in strength, but exhibits a spectral index of α = −0.88 ± 0.10. The falling component is thus consistent with nonthermal, optically thin gyrosynchrotron radiation with a corresponding power-law index similar to flares from AU Mic. While a flat component may arise from thermal, optically thin free–free emission, the observed flux density and inferred mass-loss rate are both too large compared to previous stellar wind and X-ray emission theory and models, necessitating an alternative explanation. This flat component instead matches well with an optically thick gyroresonance component integrated over multiple source regions such that the composite spectra are reasonably flat. The persistence of these components across the rotational period suggests multiple source regions, which may help explain changes in flux density and persistent high-energy electrons.