  We report our observation of the short GRB 170817A, associated to the binary
neutron star merger event GW 170817, perfomed in the X-ray band with XMM-Newton
135 d after the event (on the 29th December 2017). We find evidence for a
flattening of the X-ray light curve with respect to the previously observed
brightening. This is also supported by a nearly simultaneous optical Hubble
Space Telescope and successive X-ray Chandra and low-frequency radio
observations recently reported in the literature. Since the optical-to-X-ray
spectral slope did not change with respect to previous observations, we exclude
that the change in the temporal evolution of the light curve is due to the
passage of the cooling frequency: its origin must be geometric or dynamical. We
interpret all the existing afterglow data with two models: i) a structured jet
and ii) a jet-less isotropic fireball with some stratification in its radial
velocity structure. Both models fit the data and predict that the radio flux
must decrease simultaneously with the optical and the X-ray one, making hard to
distinguish between them at the present stage. Polarimetric measures and the
rate of short GRB-GW association in future LIGO/Virgo runs will be key to
disentangle these two geometrically different scenarios.
