Cooling off with a kilonova - lower limit on the expansion velocity of GW170817

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 ∼15 days, and a non-thermal synchrotron emission that has continued to rise ∼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 Γ > 2.1 at 9 days. This lower limit is model independent and relies directly on the observed quantities, providing additional robust evidence to the relativistic motion in this event at early times.