The evolution of the warm absorber reveals a shocked outflow in the
  narrow line Seyfert 1 galaxy IRAS 17020+4544
  We present the analysis of grating spectra of the Narrow Line Seyfert 1
Galaxy IRAS 17020+4544 observed by XMM-Newton in 2004 and 2014. In a previous
work on these data, we reported the discovery of a multi-component ultra-fast
outflow that is capable of producing feedback in the host galaxy. We also
reported the presence of a slow, multi-phase warm absorber. In this follow-up
paper, we confirm that this low velocity absorber can be modeled by four layers
of ionized gas. When crossing our line-of-sight, this gas presents peculiar
changes along the 10-yr time scale elapsed between the two observations
obtained by XMM-Newton. While two of such components are almost stationary, the
other two are found inflowing and outflowing with significant variations in
velocity and ionization between 2004 and 2014. The luminosity and spectral
shape of the central source remain practically unvaried. We propose that the
presence of the fast wind and of the variable warm absorber can be interpreted
in the framework of a `shocked outflow', where the peculiar variability pattern
of the low-velocity components might arise from instabilities in the shocked
gas.
