A vectorial problem with thin free boundary

We consider the vectorial analogue of the thin free boundary problem introduced by Caffarelli et al. (JEur math Soc 12:1151-1179, 2010) as a realization of a nonlocal version of the classical Bernoulli problem. We study optimal regularity, nondegeneracy, and density properties of local minimizers. Via a blow-up analysis based on a Weiss type monotonicity formula, we show that the free boundary is the union of a "regular" and a "singular" part. Finally we use a viscosity approach to prove C-1,C-alpha regularity of the regular part of the free boundary.