Although ultracool dwarfs ( UCDs ) are now known to generate and dissipate strong magnetic fields , a clear understanding of the underlying dynamo is still lacking .
We have performed X-ray and radio observations of seven UCDs in a narrow range of spectral type ( M6.5–M9.5 ) but spanning a wide range of projected rotational velocities ( v \sin i \approx 3 –40 km s ^ { -1 } ) .
We have also analyzed unpublished archival Chandra observations of four additional objects .
All of the newly-observed targets are detected in the X-ray , while only one is detected in the radio , with the remainder having sensitive upper limits .
We present a database of UCDs with both radio and X-ray measurements and consider the data in light of the so-called Güdel-Benz relation ( GBR ) between magnetic activity in these bands .
Some UCDs have very bright radio emission and faint X-ray emission compared to what would be expected for rapid rotators , while others show opposite behavior .
We show that UCDs would still be radio-over-luminous relative to the GBR even if their X-ray emission were at standard rapid-rotator ‘ ‘ saturation ’ ’ levels .
Recent results from Zeeman-Doppler imaging and geodynamo simulations suggest that rapidly-rotating UCDs may harbor a bistable dynamo that supports either a stronger , axisymmetric magnetic field or a weaker , non-axisymmetric field .
We suggest that the data can be explained in a scenario in which strong-field objects obey the GBR while weak-field objects are radio-over-luminous and X-ray-under-luminous , possibly because of a population of gyrosynchrotron-emitting coronal electrons that is continuously replenished by low-energy reconnection events .