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

Abstract

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 ${F}_{\nu }\propto {t}^{-1.95\pm 0.15}$ 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 $n\leqslant 9.6\times {10}^{-3}\,{\mathrm{cm}}^{-3}$. 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 ${E}_{0}={1.5}_{-1.1}^{+3.6}\times {10}^{49}\,\mathrm{erg}$ (${E}_{\mathrm{iso}}={2.1}_{-1.5}^{+6.4}\times {10}^{52}\,\mathrm{erg}$) and jet opening angle ${\theta }_{0}={5.9}_{-0.7}^{+1.0}\,\deg $ with characteristic Lorentz factor ${{\rm{\Gamma }}}_{j}={163}_{-43}^{+23}$, expanding in a low-density medium with ${n}_{0}={2.5}_{-1.9}^{+4.1}\times {10}^{-3}\,{\mathrm{cm}}^{-3}$ and viewed ${\theta }_{\mathrm{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 ${\epsilon }_{{\rm{e}}}={0.18}_{-0.13}^{+0.30}$ and ${\epsilon }_{{\rm{B}}}={2.3}_{-2.2}^{+16.0}\times {10}^{-3}$. Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of ≥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 ${E}_{k}^{\mathrm{KN}}\propto {({\rm{\Gamma }}\beta )}^{-\alpha }$ 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.

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