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

TL;DR

This study analyzes AU Mic's radio quiescent emission across 12-25 GHz, revealing two components: a variable nonthermal gyrosynchrotron emission and a stable flat component likely from gyroresonance, indicating multiple active regions.

Contribution

It identifies and characterizes two distinct radio emission components in AU Mic, providing new insights into stellar magnetic activity and emission mechanisms.

Findings

The quiescent spectrum has a falling component with spectral index -0.88.

A flat component remains steady at 0.64 mJy, likely from gyroresonance.

Multiple source regions are suggested by persistent emission components.

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 October 2018. Using Ku-band data (12 to 18 GHz) from the Very Large Array and K-band data (17 to 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 $\pm$ 0.14 mJy. The falling component varies in strength, but exhibits a spectral index of $\alpha$ = $-0.88 \pm 0.10$. 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.