We report the results of a radio monitoring program conducted at the Australia Telescope Compact Array to search for quiescent and flaring emission from seven nearby Southern late-type M and L dwarfs .
Two late-type M dwarfs , the M7 V LHS 3003 and the M8 V DENIS 1048 - 3956 , were detected in quiescent emission at 4.80 GHz .
The observed emission is consistent with optically thin gyrosynchrotron emission from mildly relativistic ( \sim 1–10 keV ) electrons with source densities n _ { e } \lesssim 10 ^ { 9 } cm ^ { -3 } in B \gtrsim 10 G magnetic fields .
DENIS 1048 - 3956 was also detected in two spectacular , short-lived flares , one at 4.80 GHz ( peak f _ { \nu } = 6.0 \pm 0.8 mJy ) and one at 8.64 GHz ( peak f _ { \nu } = 29.6 \pm 1.0 mJy ) approximately 10 minutes later .
The high brightness temperature ( T _ { B } \gtrsim 10 ^ { 13 } K ) , short emission period ( \sim 4-5 minutes ) , high circular polarization ( \sim 100 % ) , and apparently narrow spectral bandwidth of these events imply a coherent emission process in a region of high electron density ( n _ { e } \sim 10 ^ { 11 } - 10 ^ { 12 } cm ^ { -3 } ) and magnetic field strength ( B \sim 1 kG ) .
If the two flare events are related , the apparent frequency drift in the emission suggests that the emitting source either moved into regions of higher electron or magnetic flux density ; or was compressed , e.g. , by twisting field lines or gas motions .
This emission may be related to a recent optical flare from this source that exhibited indications of chromospheric mass motion .
The quiescent fluxes from the radio-emitting M dwarfs are too bright to support the Güdel-Benz empirical radio/X-ray relations , confirming a trend previously noted by Berger et al .
The violation of these relations is symptomatic of a divergence in magnetic emission trends at and beyond spectral type M7/M8 , where relative X-ray and H \alpha emission drops precipitously while relative radio emission appears to remain constant or possibly increases .
With an apparent decline in chromospheric/coronal heating , the origin of hot coronal plasmas around ultracool dwarfs remains uncertain , although external sources , such as accretion from a residual disk or tidally distorted companions , remain possibilities worth exploring .