2011ApJ...741...27M

We present multi-epoch radio and optical observations of the M7 dwarf 2MASS J13142039+1320011. We detect a ∼1 mJy source at 1.43, 4.86, 8.46, and 22.5 GHz, making it the most luminous radio emission over the widest frequency range detected from an ultracool dwarf to date. A 10 hr Very Large Array observation reveals that the radio emission varies sinusoidally with a period of 3.89±0.05 hr, and an amplitude of ~30% at 4.86 GHz and ~20% at 8.46 GHz. The periodicity is also seen in circular polarization, where at 4.86 GHz the polarization reverses helicity from left- to right-handed in phase with the total intensity. An archival detection in the Faint Images of the Radio Sky at Twenty Centimeters survey indicates that the radio emission has been stable for at least a decade. We also detect periodic photometric variability in several optical filters with a period of 3.79 hr and measure a rotation velocity of vsin i = 45±5 km/s, in good agreement with the radio and optical periods. The subtle difference in radio and optical periods may be due to differential rotation, with ΔΩ ~ 1 rad/day between the equation and poles. The period and rotation velocity allow us to place a lower limit on the radius of the source of ≳ 0.13R_{x2609}, about 30% larger than theoretical expectations. The properties of the radio emission can be explained with a simple model of a magnetic dipole misaligned relative to the stellar rotation axis, with the sinusoidal variations and helicity reversal due to the rotation of the magnetic poles relative to our line of sight. The long-term stability of the radio emission indicates that the magnetic field (and hence the dynamo) is stable on a much longer timescale than the convective turnover time of ∼0.2 yr. If the radio emission is due to gyrosynchrotron emission the inferred magnetic field strength is ∼0.1 kG, while the electron cyclotron maser process requires a field of at least 8 kG.