We present Chandra and VLA observations of GW 170817 at \sim 521 - 743 days post merger , and a homogeneous analysis of the entire Chandra dataset .
We find that the late-time non-thermal emission follows the expected evolution of an off-axis relativistic jet , with a steep temporal decay F _ { \nu } \propto t ^ { -1.95 \pm 0.15 } and power-law spectrum F _ { \nu } \propto \nu ^ { -0.575 \pm 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 find n \leq 9.6 \times 10 ^ { -3 } { cm ^ { -3 } } .
This measurement is independent from inferences based on 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 } erg ( E _ { iso } = 2.1 _ { -1.5 } ^ { +6.4 } \times 10 ^ { 52 } erg ) , jet opening angle \theta _ { 0 } = 5.9 ^ { +1.0 } _ { -0.7 } 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 } { cm ^ { -3 } } and viewed \theta _ { obs } = 30.4 ^ { +4.0 } _ { -3.4 } deg off-axis .
The synchrotron emission originates from a power-law distribution of electrons with index p = 2.15 ^ { +0.01 } _ { -0.02 } .
The shock microphysics parameters are constrained to \epsilon _ { e } = 0.18 _ { -0.13 } ^ { +0.30 } and \epsilon _ { B } = 2.3 _ { -2.2 } ^ { +16.0 } \times 10 ^ { -3 } .
Furthermore , we investigate the presence of X-ray flares and find no statistically significant evidence of \geq 2.5 \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 \leq 6 are disfavored .
Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties .