An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009 detected a photoionised outflow with a complex absorption line velocity structure and a broad correlation of velocity with ionisation parameter , shown in Pounds et al .
( 2011 ) to be consistent with a highly ionised , high velocity wind running into the interstellar medium or previous ejecta , losing much of its kinetic energy in the resultant strong shock .
In the present paper we examine the FeK spectral region in more detail and find support for two distinct velocity components in the highly ionised absorber , with values corresponding to the putative fast wind ( \sim 0.12c ) and the post-shock flow ( v \sim 5000-7000 km s ^ { -1 } ) .
The Fe K absorption line structure is seen to vary on a orbit-to-orbit timescale , apparently responding to both a short term increase in ionising flux and - perhaps more generally - to changes in the soft X-ray ( and simultaneous UV ) luminosity .
The latter result is particularly interesting in providing independent support for the existence of shocked gas being cooled primarily by Compton scattering of accretion disc photons .
The Fe K emission is represented by a narrow fluorescent line from near-neutral matter , with a weak red wing modelled here by a relativistic diskline .
The narrow line flux is quasi-constant throughout the 45-day 2009 campaign , but is resolved , with a velocity width consistent with scattering from a component of the post-shock flow .
Evidence for a P Cygni profile is seen in several individual orbit spectra for resonance transitions in both Fe XXV and Fe XXVI .