  We report on a detailed study of the Fe K emission/absorption complex in the
nearby, bright Seyfert 1 galaxy Mrk 509. The study is part of an extensive
XMM-Newton monitoring consisting of 10 pointings (~60 ks each) about once every
four days, and includes also a reanalysis of previous XMM-Newton and Chandra
observations. Mrk 509 shows a clear (EW=58 eV) neutral Fe Kalpha emission line
that can be decomposed into a narrow (sigma=0.027 keV) component (found in the
Chandra HETG data) plus a resolved (sigma=0.22 keV) component. We find the
first successful measurement of a linear correlation between the intensity of
the resolved line component and the 3-10 keV flux variations on time-scales of
years down to a few days. The Fe Kalpha reverberates the hard X-ray continuum
without any measurable lag, suggesting that the region producing the resolved
Fe Kalpha component is located within a few light days-week (r<~10^3 rg) from
the Black Hole (BH). The lack of a redshifted wing in the line poses a lower
limit of >40 rg for its distance from the BH. The Fe Kalpha could thus be
emitted from the inner regions of the BLR, i.e. within the ~80 light days
indicated by the Hbeta line measurements. In addition to these two neutral Fe
Kalpha components, we confirm the detection of weak (EW~8-20 eV) ionised Fe K
emission. This ionised line can be modeled with either a blend of two narrow
FeXXV and FeXXVI emission lines or with a single relativistic line produced, in
an ionised disc, down to a few rg from the BH. Finally, we observe a
weakening/disappearing of the medium and high velocity high ionisation Fe K
wind features found in previous XMM-Newton observations. This campaign has made
possible the first reverberation measurement of the resolved component of the
Fe Kalpha line, from which we can infer a location for the bulk of its emission
at a distance of r~40-1000 rg from the BH.
