A new window for magnetic activity on Ultracool dwarfs: eROSITA & a synergistic multiwavelength approach

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Ultracool dwarfs (UCDs) are defined as objects with spectral type M7 and later. This group spans the hydrogen-burning mass limit (at ~ 0.07 solar masses) and it comprises both very low-mass stars and brown dwarfs. The term UCD avoids misnoming as the mass of such objects is unknown in most cases and, therefore, for a given UCD it is unclear if it is able to burn hydrogen and thus whether it is a star or a brown dwarf.As a result of their faintness the study of magnetic activity on UCDs has advanced only duringthe last ~15 yrs. This uncovered a puzzling behavior: the activity of UCDs might be characterized by a transition from stellar-like (X-ray flaring and radio-faint) to planetary-like (radio-bursting and X-ray faint) magnetic processes. The physical origin of these differences has not yet been identified but the magnetic field strength and morphology that is probably ruled by the rotation rate likely plays a major role. Verifying or refuting the previous evidence for the dichotomy of magnetic activity near the substellar boundary and systematically searching for its origin through a comprehensive multi-wavelength study is the aim of this project. The advent of the (nearly) all-sky space missions eROSITA and TESS opens a new window to UCDs, providing for the first time statistical samples of coronal X-ray emission and photospheric optical activity measures and surface rotation periods. eROSITA will increase the number of X-ray detected UCDs by at least a factor of 10 and it will enable a systematic study of flare variability thanks to its multi-epoch data (8 visits over 4 years). TESS provides optical flares and rotation periods that -- according to previous studies -- appear to be crucial for determining the type of activity in UCDs. This will be complemented by an in-depth study of UCDs for which the applicant of this funding request has collected multi-wavelength observations of all critical parameters for activity (X-rays, radio and rotation). Such extremely high sensitivity observations with XMM-Newton and the JVLA for a carefully selected sample of UCDs with TESS rotation periods is crucial for unvealing the origin of the transition in the activity behavior.