  We use polarized inelastic neutron scattering (INS) to study spin excitations
in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF)
order along the a-axis of the orthorhombic structure and NaFe0.935Co0.045As
(C45) without AF order. In previous unpolarized INS work, spin excitations in
C15 were found to have a dispersive sharp resonance near Er1=3.25 meV and a
broad dispersionless mode at Er2=6 meV. Our neutron polarization analysis
reveals that the dispersive resonance in C15 is highly anisotropic and
polarized along the a- and c-axis, while the dispersionless mode is isotropic
similar to that of C45. Since the a-axis polarized spin excitations of the
anisotropic resonance appear below Tc, our data suggests that the itinerant
electrons contributing to the magnetism are also coupled to the
superconductivity.
