2019ApJ...886L..17H

We present Chandra and Very Large Array 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 nonthermal emission follows the expected evolution of an off-axis relativistic jet, with a steep temporal decay F_ν∝t^-1.95±0.15^ and power-law spectrum F_ν∝ν^-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 find n≤ 9.6×10^–3cm^–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 E₀=1.5_–1.1^+3.6×10⁴⁹erg (E_iso=2.1_–1.5^+6.4×10⁵²erg) and jet opening angle θ₀=5.9_–0.7^+1.0°with characteristic Lorentz factor Γ_j=163_–43⁺²³, expanding in a low-density medium with n₀=2.5_–1.9^+4.1×10^–3cm^–3 and viewed θ_obs=30.4_–3.4^+4.0°off-axis. The synchrotron emission originates from a power-law distribution of electrons with index p=2.15_–0.02^+0.01. The shock microphysics parameters are constrained to ε_e=0.18_–0.13^+0.30 and ε_B=2.3_–2.2^+16.0×10^–3. Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of >=2.5σ 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∝(Γβ)^-α^ into the environment, finding that shallow stratification indexes α <= 6 are disfavored. Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties.