A short radio campaign on AU Mic has found that its apparently quiet emission was changing even during selected quiescent intervals, while two clear flares erupted during the same observing run. The pattern is consistent with a picture in which many small, unresolved flares may repeatedly inject energetic electrons while magnetic trapping helps sustain the radio emission. The authors present that as an interpretation of the observations, not as proof that micro-flares power all of AU Mic’s radio emission.
The study focused on AU Mic, a single 22-million-year-old dM1e star, using sequential observations from the Very Large Array. The radio measurements covered the Ku band from 12 to 18 gigahertz and the K band from 18 to 26 gigahertz, with about 1.5 hours of on-source integration in each band.
The star was restless even between flares
The researchers selected intervals they considered quiescent, meaning they were not dominated by an obvious flare. Even there, the data showed intrinsic variability with a probability greater than 99.9% in each band. In plain terms, the star’s baseline radio output was not truly steady.
To track those changes, the team fit background-subtracted radio measurements in 30-second bins. It also separately fit the two circular-polarization channels in one-minute bins to derive the total intensity and circularly polarized components.
Averaged over time, the quiescent emission was about 0.4 millijanskys. Its spectrum peaked around 17 gigahertz and then fell with an optically thin spectral index of about minus 0.6. The researchers describe that shape as best explained by gyrosynchrotron radiation, a form of radio emission associated with energetic electrons in magnetic fields.
The spectral peak was estimated with repeated Monte Carlo fits using a curved log-parabola model, with an allowance for systematic flux differences between the two sequentially observed bands. Because the bands were not recorded simultaneously, the result is a time-averaged spectrum that assumes the source behavior was comparable across the observations.
Two flares offered a closer look
Across three contiguous hours on source, the observations captured two flares and two additional marginal events, corresponding to roughly one flare per hour. That rate comes from a single observing run of a single star, so it is not a general rate for active M dwarfs.
The clear flares rose quickly and faded more slowly. Fits to their light curves gave decay-to-rise timescale ratios of 3.82 in the Ku band and 3.16 in the K band. The profiles were modeled with separate rising and falling exponentials to estimate the flare timing, peak flux and characteristic rise and decay times.
The Ku-band flare was still optically thick during its rise and at its peak, meaning the emitting region was not yet transparent to its own radio radiation. Its spectral peak therefore lay above 18 gigahertz, beyond the lower edge of the K band.
A small active region, according to the model
Modeling the radio source produced an estimated magnetic field of roughly 1 kilogauss, with the active region covering less than 0.5% of the stellar surface. The analysis also reported a large instantaneous total kinetic energy in the emitting electrons, written in the study as about 10^28 erg.
The authors connect an inferred electron power-law index of about 2 with near-continuous electron injection. In their preferred interpretation, surface micro-flares repeatedly supply the particles, while magnetic trapping helps keep them radiating for longer. The slow flare decays, low polarization and small modeled source coverage are presented as consistent with that scenario.
The polarization measurements were broadly quiet: neither the quiescent intervals nor the flares showed significant circular polarization. One burst lasting less than three seconds was slightly polarized, but its true flux, polarization and physical mechanism could not be resolved, leaving it separate from the study’s main flare interpretation.
What the observations cannot settle
The main uncertainty is the study’s narrow scope. It examines one star in one radio observing run. The proposed micro-flare picture may describe AU Mic during these observations, but the data do not establish that the same process is common across active M-dwarf coronae or that it causes the star’s entire quiescent radio output.
The source properties are also model-dependent, especially the magnetic field estimate. The authors caution that the magnetic field can vary through a flare loop and say the reported values should be treated as tentative.
Because only one star was observed, the proposed pattern still needs testing across multiple observing epochs and other active M dwarfs. Broader simultaneous radio coverage above 26 gigahertz could better constrain the spectrum, while the short polarized burst remains an unresolved question of its own.
The document is a preprint, arXiv:2608.25400v1, dated 26 August 2026, and says it was submitted to the Astrophysical Journal. The acknowledgements associate the observations with JWST program 5311.
Paper data and sources
Original title: Broadband 12-26 GHz Radio Radiation Reveals Evidence for Micro-flares on AU Mic
Authors: Isaiah I. Tristan, Rachel A. Osten, Yuta Notsu et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: 10.3847/1538-4357/ae9d50
Original paper · Full text