Description
X-ray timing and broadband spectroscopy have been the main methods for studying the rich and
complex phenomenology of neutron stars with weak magnetic fields for a long time. Nowadays,
X-ray polarimetry offers the missing, independent set of observables that are directly relevant to
comprehending the geometry and nature of the emission regions: polarization degree and angle.
This allows breaking degeneracies because different geometries and physical conditions might
yield equivalent spectral and timing signatures. Some of the main results obtained in this frame
by using the Imaging X-ray Polarimetry Explorer (IXPE) will be reported in this talk. In particular,
IXPE observations of Z sources have shown that the polarization properties are not constant but
evolve significantly with spectral state. A common trend has emerged in all the Z sources studied
so far: the polarization decreases markedly during the transition from the horizontal branch (HB)
to the normal branch (NB), indicating substantial changes as the accretion flow evolves.