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

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. LaskarB. D. Metzger, M. Nicholl, K. Paterson, D. Radice, L. Sironi, G. Terreran, V. A. Villar, P. K.G. Williams, X. Xie, J. Zrake

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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 Fν ∝ t-1.95±0.15 and power-law spectrum Fν ∝ 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 (Eiso = 2.1-1.5 +6.4 × 1052 erg) and jet opening angle θ0 = 5.9-0.7 +1.0 deg with characteristic Lorentz factor Γj = 163-43 +23, expanding in a low-density medium with n0 = 2.5-1.9 +4.1 × 10-3 cm-3 and viewed θobs = 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 ϵe = 0.18-0.13 +0.30 and ϵB = 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 Ek KN ∝ (Γ β) 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.

Original languageEnglish (US)
Article numberL17
JournalAstrophysical Journal Letters
Volume886
Issue number1
DOIs
StatePublished - Nov 20 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Hajela, A., Margutti, R., Alexander, K. D., Kathirgamaraju, A., Baldeschi, A., Guidorzi, C., Giannios, D., Fong, W., Wu, Y., Macfadyen, A., Paggi, A., Berger, E., Blanchard, P. K., Chornock, R., Coppejans, D. L., Cowperthwaite, P. S., Eftekhari, T., Gomez, S., Hosseinzadeh, G., ... Zrake, J. (2019). 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. Astrophysical Journal Letters, 886(1), [L17]. https://doi.org/10.3847/2041-8213/ab5226