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

A. Hajela; R. Margutti; K. D. Alexander; A. Kathirgamaraju; A. Baldeschi; C. Guidorzi; D. Giannios; W. Fong; Y. Wu; A. MacFadyen; A. Paggi; E. Berger; P. K. Blanchard; R. Chornock; D. L. Coppejans; P. S. Cowperthwaite; T. Eftekhari; S. Gomez; G. Hosseinzadeh; T. Laskar; B. D. Metzger; M. Nicholl; K. Paterson; D. Radice; L. Sironi; G. Terreran; V. A. Villar; P. K. G. Williams; X. Xie; J. Zrake

doi:10.3847/2041-8213/ab5226

paper

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

arXiv:1909.06393 · doi:10.3847/2041-8213/ab5226

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 $F_{ν} \propto t^{- 1.95 \pm 0.15}$ and a simple power-law spectrum $F_{ν} \propto ν^{- 0.575 \pm 0.007}$ . 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 $n \leq 9.6 \times 1 0^{- 3} c m^{- 3}$ . 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 $E_{0} = 1. 5_{- 1.1}^{+ 3.6} \times 1 0^{49} erg$ ( $E_{i so} = 2. 1_{- 1.5}^{+ 6.4} \times 1 0^{52} erg$ ), jet opening angle $θ_{0} = 5. 9_{- 0.7}^{+ 1.0} deg$ with characteristic Lorentz factor $Γ_{j} = 16 3_{- 43}^{+ 23}$ , expanding in a low-density medium with $n_{0} = 2. 5_{- 1.9}^{+ 4.1} \times 1 0^{- 3} c m^{- 3}$ and viewed $θ_{o b s} = 30. 4_{- 3.4}^{+ 4.0} deg$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p = 2.1 5_{- 0.02}^{+ 0.01}$ . The shock microphysics parameters are constrained to $ε_{e} = 0.1 8_{- 0.13}^{+ 0.30}$ and $ε_{B} = 2. 3_{- 2.2}^{+ 16.0} \times 1 0^{- 3}$ . We investigate the presence of X-ray flares and find no statistically significant evidence of $\geq 2.5 σ$ 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 $E_{k}^{K N} \propto (Γ β)^{- α}$ into the environment, finding that shallow stratification indexes $α \leq 6$ are disfavored.

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

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

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