Two Years of Nonthermal 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 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^-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 × 10⁴⁹ erg (E_iso = 2.1_-1.5 ^+6.4 × 10⁵² erg) and jet opening angle θ₀ = 5.9_-0.7 ^+1.0 deg with characteristic Lorentz factor Γ_j = 163_-43 ⁺²³, expanding in a low-density medium with n₀ = 2.5_-1.9 ^+4.1 × 10^-3 cm^-3 and viewed θ_obs = 30.4_-3.4 ^+4.0 deg 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 ∝ (Γ β)^-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.