  GW170817 was the first detection of a binary neutron star merger via
gravitational waves. The event was observed over a wide range of the
electromagnetic spectrum, revealing a thermal kilonova dominating the optical
signal during the first $\sim$15 days, and a non-thermal synchrotron emission
that has continued to rise $\sim$200 days post-merger, dominating the radio and
x-ray emission. At early times, when the kilonova is still dominant, the
synchrotron emitting electrons can efficiently cool by up-scattering the
kilonova photos through inverse-Compton. Yet, the cooling frequency is not
observed up to the X-ray band. This can only be explained if the source is
moving at least at a mildly relativistic velocity. We find a lower limit on the
source's bulk Lorentz factor of $\Gamma > 2.1$ at $9$ days. This lower limit is
model independent, and relies directly on the observed quantities, providing an
additional robust evidence to the relativistic motion in this event at early
times.
