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 $F_{ \nu}\propto t^{ -1.95\pm0.15}$ and a simple power-law spectrum $F_{ \nu}\propto \nu^{ -0.575\pm0.007}$. 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 $n\le 9.6 \times 10^{ -3}\,\rm{ cm^{ -3}}$. 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 $E_0 = 1.5_{ -1.1}^{ +3.6}\times 10^{ 49}\,\rm{ erg}$ ($E_{ iso}= 2.1_{ -1.5}^{ +6.4}\times10^{ 52}\, \rm{ erg}$), jet opening angle $\theta_{ 0}= 5.9^{ +1.0}_{ -0.7}\,\rm{ deg}$ with characteristic Lorentz factor $\Gamma_j = 163_{ -43}^{ +23}$, expanding in a low-density medium with $n_0 = 2.5_{ -1.9}^{ +4.1} \times 10^{ -3}\, \rm{ cm^{ -3}}$ and viewed $\theta_{ obs} = 30.4^{ +4.0}_{ -3.4}\, \rm{ deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{ +0.01}_{ -0.02}$. The shock microphysics parameters are constrained to $\epsilon_{ \rm{ e}} = 0.18_{ -0.13}^{ +0.30}$ and $\epsilon_{ \rm{ B}}=2.3_{ -2.2}^{ +16.0} \times 10^{ -3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\ge2.5\sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{ KN}\propto (\Gamma\beta)^{ -\alpha}$ into the environment, finding that shallow stratification indexes $\alpha\le6$ are disfavored.
Categories: astro-ph.HE