X-ray evidence for a mildly relativistic and variable outflow in the luminous Seyfert 1 galaxy Mrk 509

Context. There is growing evidence for the presence of blueshifted Fe K absorption lines in a number of radio-quiet AGNs and QSOs. These may be fundamental to probe flow dynamics near supermassive black holes. Aims. Here we aim to verify and better characterise the existence of such Fe K absorption at similar to 8-10 keV in the luminous Seyfert 1 galaxy Mrk 509, one of the most promising target for these studies. Methods. We present a comprehensive spectral analysis of the six XMM-Newton observations of the source (for a total of similar to 200 ks), focusing on a detailed and systematic search for absorption features in the high-energy data. Results. We detect several absorption features at rest-frame energies similar to 8-8.5 keV and similar to 9.7 keV. The lines are consistent with being produced by H-like iron K alpha and K beta shell absorptions associated with an outflow with a mildly relativistic velocity of similar to 0.14-0.2 c. The lines are found to be variable in energy and, marginally, in intensity, implying variations in either the column density, geometry and/or ionization structure of the outflow are common.