Searching for radio emission from stellar wind–magnetosphere interaction or co-rotation breakdown in brown dwarfs

ABSTRACT With the improvements in radio interferometry sensitivity, the quest for coherent radio emission from exoplanets and ultra-cool dwarfs, which is indicative of their magnetic fields, has gained significant momentum in recent years. We investigated the relatively unexplored possibility of radio emission from wide-orbit brown dwarf companions, which may radiate through rapid rotation, as in isolated ultra-cool dwarfs, or via interactions between their extended magnetospheres and the host star’s wind. We analysed $\sim 60$ h of Upgraded Giant Metrewave Radio Telescope and Karl G. Jansky Very Large Array data for a set of well-characterized systems previously unobserved at 0.3–2 GHz. The targets include companions orbiting the G-type stars HD 26161 and BD-004475, the K-type HD 153557A and $\nu$ Oph, and the M dwarfs GJ 3626 and 2MJ01225093-2439505. No detections were obtained with 3$\sigma$ upper limits down to $\sim 25\, \mu$Jy beam−1 in Stokes V in the best cases. The light-curve analysis also revealed no evidence of short ($\gtrsim$ minutes), intense ($\gtrsim$ mJy) radio bursts. The upper limits provide tentative constraints on model parameters. However, the effects of model uncertainties, limited observational coverage, and intrinsic variability or beaming of the emission must be considered. The improvement in sensitivity of the next-generation radio interferometers will likely allow us to go below the expected flux range over a much larger range of free parameters.