  Brown dwarfs are classified as objects which are not massive enough to
sustain nuclear fusion of hydrogen, and are distinguished from planets by their
ability to burn deuterium. Old (>10 Myr) brown dwarfs are expected to possess
short-lived magnetic fields and, since they no longer generate energy from
collapse and accretion, weak radio and X-ray emitting coronae. Several efforts
have been undertaken in the past to detect chromospheric activity from the
brown dwarf LP944-20 at X-ray and optical wavelengths, but only recently an
X-ray flare from this object was detected. Here we report on the discovery of
quiescent and flaring radio emission from this source, which represents the
first detection of persistent radio emission from a brown dwarf, with
luminosities that are several orders of magnitude larger than predicted from an
empirical relation between the X-ray and radio luminosities of many stellar
types. We show in the context of synchrotron emission, that LP944-20 possesses
an unusually weak magnetic field in comparison to active dwarf M stars, which
might explain the null results from previous optical and X-ray observations of
this source, and the deviation from the empirical relations.
