Ultracool dwarfs , the least-massive contributors to the stellar mass function , exhibit striking magnetic properties that are inconsistent with trends for more massive stars .
Here , we present the widest-band radio observations to date of an ultracool dwarf , DENIS-P J104814.9 - 395604 , in four 2 GHz bandwidths between wavelengths of 1 cm and 10 cm .
These data were obtained with the Australia Telescope Compact Array using the new Compact Array Broadband Backend instrument .
We detected a stable negatively-sloped power-law spectrum in total intensity , with spectral index \alpha = 1.71 \pm 0.09 .
Circular polarization fractions between 0.25 and 0.4 were found at the low-frequency end of our detection band .
We interpret these results as indicative of gyrosynchrotron emission .
We suggest that the radio emission originates from beyond the co-rotation radius , R _ { C } , of the star .
Adopting this model , we find R _ { C } between 1.2 - 2.9 R _ { * } , and a non-thermal electron density and magnetic field strength between 10 ^ { 5 } -10 ^ { 7.2 } cm ^ { -3 } and 70 - 260 G respectively at R _ { C } .
The model accounts for the violation of the Güdel-Benz relation between X-ray and radio luminosities of low-mass stars by DENIS-P J104814.9 - 395604 .