X-ray emission near the substellar limit: The

We have carried out an extensive search for X-ray emission from young, very low-mass objects near and beyond the substellar limit, making use of archived ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf candidates in the young σ Orionis and Taurus-Auriga associations. In σ Ori we identify three Brown Dwarf candidates with X-ray sources; in Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list published earlier. We combine this data with all previously X-ray detected Brown Dwarfs and Brown Dwarf candidates in young stellar associations and star forming regions to perform a study of stellar activity parameters on the as yet largest sample of young, very low mass objects. A similar relation between X-ray and bolometric luminosity, and Hα emission, respectively, as is known for T Tauri stars seems to hold for young objects down to the substellar limit, too. No signs for a change in X-ray activity are found on the transition to substellar masses