An extended XMM-Newton observation of the Seyfert galaxy NGC 4051 - I. Evidence for a shocked outflow

An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 has revealed a rich absorption line spectrum indicating the presence of a photoionized outflow with a wide range of velocities and ionization parameter. At low continuum fluxes an emission line spectrum is well defined with both narrow and broad emission components of several abundant metal ions. The absorption line velocity structure and a broad correlation of velocity with ionization parameter are consistent with an outflow scenario where a highly ionized, high-velocity wind, perhaps launched during intermittent super-Eddington accretion, runs into the interstellar medium or previous ejecta, losing much of its kinetic energy in the resultant strong shock. We explore the possibility that a quasi-constant soft X-ray component may be evidence of post-shock cooling. This revised view of active galactic nuclei (AGN) outflows is consistent with multiple minor Eddington accretion episodes creating a momentum-driven feedback linking black hole and host galaxy growth.