Quasi-Quiescent Radio Emission from the First Radio-Emitting T Dwarf

Radio detections of ultracool dwarfs provide insight into their magnetic fields and the dynamos that maintain them, especially at the very bottom of the main sequence, where other activity indicators dramatically weaken. Until recently, the coolest brown dwarf detected in the radio was only of spectral type L3.5, but this has changed with the Arecibo detection of rapid polarized flares from the T6.5 dwarf 2MASS J10475385+2124234. Here, we report the detection of quasi-quiescent radio emission from this source at 5.8 GHz using the Karl G. Jansky Very Large Array. The luminosity is νL_ν = (1.3 +- 0.4) x 10^22 erg/s, a factor of ~100 times fainter than the Arecibo flares. Our detection is the lowest-luminosity yet achieved for an ultracool dwarf. Although the emission is fully consistent with being steady, unpolarized, and broadband, we find tantalizing hints for variability. We exclude the presence of short-duration flares as seen by Arecibo, although this is not unexpected given estimates of the duty cycle. Follow-up observations of this object will offer the potential to constrain its rotation period, electron density, and the strength and configuration of the magnetic field. Equally important, follow-up will address the question of whether the electron cyclotron maser instability, which is thought to produce the flares seen by Arecibo, also operates in the very different parameter regime of the emission we detect, or whether instead this ultracool dwarf exhibits both maser and gyrosynchrotron radiation, potentially originating from substantially different locations.