Ultracool dwarfs (UCDs) are objects with spectral type M7 and later, at the age-dependent transition between stellar and brown dwarf regime. In the predominantly neutral photospheres of UCDs the magnetic field and the matter are expected to show poor coupling, and this should be shutting off the magnetic activity. Nevertheless, radio, Halpha and X-ray emission has occasionally been observed from UCDs.
The most peculiar feature in the multi-wavelength characteristics of UCDs is their radio/X-ray dichotomy: (a) X-ray flaring objects tend to be radio-faint and (b) radio-bursting UCDs are mostly undetected in the X-ray band. These two groups have been associated with (a) `normal' magnetic activity as observed on solar-type stars and (b) phenomena seen on the giant planets of the Solar System, respectively. For UCDs the nature of the corona is thus poorly understood, and they may represent a transition between solar-like and planetary-like magnetic field structure.
I will present several ongoing projects to characterize the multi-wavelength activity of UCDs. These studies include (i) joint XMM-Newton / JVLA observations to examine the dependence of the radio-X-ray relation on other critical parameters such as rotation rate and magnetic field strength, (ii) the serendipitous detection of an X-ray super-flare on an L dwarf, and (iii) a first look into the eFEDS field which comprises a few tens of UCDs and candidate UCDs.
|Presenter status||eROSITA_DE member|