  We study the entanglement in the ground state of a chain of free spinless
fermions with a single side-coupled impurity. We find a logarithmic scaling for
the entanglement entropy of a segment neighboring the impurity. The prefactor
of the logarithm varies continuously and contains an impurity contribution
described by a one-parameter function, while the contribution of the unmodified
boundary enters additively. The coefficient is found explicitly by pointing out
similarities with other models involving interface defects. The proposed
formula gives excellent agreement with our numerical data. If the segment has
an open boundary, one finds a rapidly oscillating subleading term in the
entropy that persists in the limit of large block sizes. The particle number
fluctuation inside the subsystem is also reported. It is analogous with the
expression for the entropy scaling, however with a simpler functional form for
the coefficient.
