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

Citation:

Hajela A, Margutti R, Alexander K D, Kathirgamaraju A, Baldeschi A, Guidorzi C, Giannios D, Fong W, Wu Y, MacFadyen A, et al. 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. The Astrophysical Journal Letters. 2019;886 (1) :L17.

Date Published:

Nov

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}ν \propto {t}-1.95+/- 0.15 and power-law spectrum {F}ν \propto {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≤slant 9.6× {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× {10}49 {erg} ({E}iso}={2.1}-1.5+6.4× {10}52 {erg}) and jet opening angle {θ }0={5.9}-0.7+1.0 \deg with characteristic Lorentz factor {{{Γ }}}j={163}-43+23, expanding in a low-density medium with {n}0={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}kKN}\propto {({{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.

Publisher's Version

Last updated on 02/16/2020