Unusual periodic modulation in the radio emission of the methane dwarf binary WISEP J101905.63+652954.2

Brown dwarfs display Jupiter-like auroral phenomena, such as rotationally modulated electron cyclotron maser radio emission. Radio observations of cyclotron maser emission can be used to measure their magnetic field strength and topology, and to deduce the presence of magnetically interacting exoplanets. Observations of the coldest brown dwarfs (spectral types T and Y) are especially intriguing, as their magnetospheric phenomena could, in fact, closely resemble those of gas-giant exoplanets. Here, we report observations made over ten epochs, amounting to 44 hours, of WISEP J101905.63+652954.2 (J1019+65, hereinafter) using the LOFAR telescope between 120 and 168 MHz. J1019+65 is a methane dwarf binary (T5.5 + T7) whose radio emission was originally detected in a single-epoch LOFAR observation to be highly circular polarised and rotationally modulated at ≈3 h. Unexpectedly, our long-term monitoring reveals an additional periodic signature at ≈0.787 h. We considered several explanations for the second period and ultimately proposed that it could be the rotationally modulated emission of the second brown dwarf in the binary, although follow-up infrared (IR) observations are necessary to confirm this hypothesis. In addition, the data also allowed us to statistically estimate the duty cycle and observed radio-loud fraction, respectively, of the 120-168 MHz cyclotron emission from methane dwarfs to be ⟨D⟩ = 0.030−0.030+0.034 and F′radio = 0.088−0.088+0.168.