  We study a quantum impurity coupled to the edge states of a two-dimensional
helical topological superconductor, i.e., to a pair of counterpropagating
Majorana fermion edge channels with opposite spin polarizations. For an
impurity described by the Anderson impurity model, we show that the problem
maps onto a variant of the interacting resonant two-level model which, in turn,
maps onto the ferromagnetic Kondo model. Both magnetic and non-magnetic
impurities are considered. For magnetic impurities, the bosonization and
numerical renormalization group analyses show that the system flows to a fixed
point with residual ln 2 entropy and we find characteristically anisotropic
static and dynamic impurity magnetic susceptibilities. For non-magnetic
impurities, the system flows to a fixed point with no residual entropy and we
find diamagnetic response at low temperatures. We comment on the
Schrieffer-Wolff transformation for problems with non-standard conduction band
continua and on the issues related to the differences in describing the
impurities by either Anderson or Kondo impurity models.
