Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement

Abstract

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 ≈ 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 We have also analyzed unpublished archival Chandra observations of four additional objects.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-rayunder-luminous, possibly because of a population of gyrosynchrotron-emitting coronal electrons that is continuously replenished by low-energy reconnection events.