Two years of non-thermal emission from the binary neutron star merger GW170817: rapid fading of the jet afterglow and first constraints on the kilonova fastest ejecta

We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay Fnu propto t^(-1.95 +- 0.15) and a simple power-law spectrum Fnu propto nu^(-0.575 +- 0.007). We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find n< 9.6 x 10^-3 cm^-3. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy E_0 = 1.5 (-1.1, +3.6) x 10^49, erg (E_iso= 2.1 (-1.5,+6.4)x10^52 erg), jet opening angle theta_0=5.9 (-0.7,+1.0) deg with characteristic Lorentz factor Gamma_j = 163 (-43,+23), expanding in a low-density medium with n_0 =2.5 (-1.9,+4)x 10^-3 cm^-3 and viewed theta_obs =30.4 (-3.4, +4.0) deg off-axis. The synchrotron emission originates from a power-law distribution of electrons with p=2.15 (-0.02,+0.01). The shock microphysics parameters are %loosely constrained to epsilon_e =0.18 (-0.13,+0.30) and epsilon_B=2.3 (-2.2,+16.0)x 10^-3. We investigate the presence of X-ray flares and find no statistically significant evidence of $le2.5sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the astest kilonova ejecta with energy E_k^(KN) propto (Gamma beta)^-alpha into the environment, finding that shallow stratification indexes alpha <=6 are disfavored.