The magnetosphere of the ultracool dwarf DENIS1048-3956

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-PJ104814.9-395604, in four 2GHz bandwidths between wavelengths of 1cm and 10cm. 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 α = 1.71 ± 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 corotation radius, R_C , of the star. Adopting this model, we find R_C between 1.2 R _* and 2.9 R _*, and a non-thermal electron density and magnetic field strength between 10⁵ and 10^7.2cm^-3 and between 70 and 260G, 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-PJ104814.9-395604.