  Periodic radio bursts from very low mass stars and brown dwarfs
simultaneously probe their magnetic and rotational properties. The brown dwarf
2MASSI J1047539+212423 (2M 1047+21) is currently the only T dwarf (T6.5)
detected at radio wavelengths. Previous observations of this source with the
Arecibo observatory revealed intermittent, 100%-polarized radio pulses similar
to those detected from other brown dwarfs, but were unable to constrain a pulse
periodicity; previous VLA observations detected quiescent emission a factor of
~100 times fainter than the Arecibo pulses but no additional events. Here we
present 14 hours of Very Large Array observations of this object that reveal a
series of pulses at ~6 GHz with highly variable profiles, showing that the
pulsing behavior evolves on time scales that are both long and short compared
to the rotation period. We measure a periodicity of ~1.77 hr and identify it
with the rotation period. This is just the sixth rotation period measurement in
a late T dwarf, and the first obtained in the radio. We detect a pulse at 10
GHz as well, suggesting that the magnetic field strength of 2M 1047+21 reaches
at least 3.6 kG. Although this object is the coolest and most rapidly-rotating
radio-detected brown dwarf to date, its properties appear continuous with those
of other such objects, suggesting that the generation of strong magnetic fields
and radio emission may continue to even cooler objects. Further studies of this
kind will help to clarify the relationships between mass, age, rotation, and
magnetic activity at and beyond the end of the main sequence, where both
theories and observational data are currently scarce.
  A video of a short talk discussing this work is at
http://youtu.be/OqCtFxcBB2s, and an experimental Web-native version of the
paper may be viewed at http://purl.org/net/pkgwpub/goreham.
