  We address the question of how interacting active systems in a
non-equilibrium steady-state respond to an external perturbation. We establish
an extended fluctuation-dissipation theorem for Active Brownian Particles (ABP)
which highlights the role played by the local violation of detailed balance due
to activity. By making use of a Markovian approximation we derive closed
Green-Kubo expressions for the diffusivity and mobility of ABP and quantify the
deviations from the Stokes-Einstein relation. We compute the linear response
function to an external force using unperturbed simulations of ABP and compare
the results with the analytical predictions of the transport coefficients. Our
results show the importance of the interplay between activity and interactions
in the departure from equilibrium linear response.
