Two years of non-thermal 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 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 $F_{\hat{I}}½\propto t^{-1.95\pm 0.15}$ and a simple power-law spectrum $F_{\hat{I}}½\propto \hat{I}{½}^{-0.575\pm 0.007}$. 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 we find $n\le 9.6\times 10^{-3}\mathrm{c}{\mathrm{m}}^{-3}$. 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 $E_0=1.5_{-1.1}^{+3.6}\times 10^{49}\mathrm{erg}$ ($E_{iso}=2.1_{-1.5}^{+6.4}\times 10^{52}\mathrm{erg}$), jet opening angle $\hat{I}{¸}_0=5.9_{-0.7}^{+1.0}\mathrm{deg}$ with characteristic Lorentz factor $\hat{I}{“}_j=163_{-43}^{+23}$, expanding in a low-density medium with $n_0=2.5_{-1.9}^{+4.1}\times 10^{-3}\mathrm{c}{\mathrm{m}}^{-3}$ and viewed $\hat{I}{¸}_{obs}=30.4_{-3.4}^{+4.0}\mathrm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15_{-0.02}^{+0.01}$. The shock microphysics parameters are constrained to $\hat{I}{\mu }_{\mathrm{e}}=0.18_{-0.13}^{+0.30}$ and $\hat{I}{\mu }_{\mathrm{B}}=2.3_{-2.2}^{+16.0}\times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\ge 2.5\ddot{I}ƒ$ 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 $E_k^{KN}\propto (\hat{I}“{\hat{I}}^2{)}^{-\hat{I}\pm }$ into the environment, finding that shallow stratification indexes $\hat{I}\pm \le 6$ are disfavored.

version accepted for publication in ApJL, 13 pages, 6 figures