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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

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
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
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.
2041-8205
L17
Hajela, A.
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Margutti, R.
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Alexander, K. D.
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Kathirgamaraju, A.
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Baldeschi, A.
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Guidorzi, C.
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Giannios, D.
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Fong, W.
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Wu, Y.
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Macfadyen, A.
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Paggi, A.
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Berger, E.
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Blanchard, P. K.
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Chornock, R.
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Coppejans, D. L.
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Eftekhari, T.
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Gomez, S.
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Hosseinzadeh, G.
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Laskar, T.
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Metzger, B. D.
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Nicholl, M.
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Paterson, K.
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Radice, D.
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Sironi, L.
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Terreran, G.
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Villar, V. A.
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Williams, P. K. G.
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Xie, X.
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Zrake, J.
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Hajela, A.
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Margutti, R.
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Alexander, K. D.
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Kathirgamaraju, A.
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Baldeschi, A.
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Guidorzi, C.
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Giannios, D.
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Fong, W.
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Wu, Y.
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Macfadyen, A.
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Paggi, A.
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Berger, E.
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Blanchard, P. K.
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Chornock, R.
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Coppejans, D. L.
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Cowperthwaite, P. S.
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Eftekhari, T.
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Gomez, S.
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Hosseinzadeh, G.
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Laskar, T.
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Metzger, B. D.
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Nicholl, M.
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Paterson, K.
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Radice, D.
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Sironi, L.
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Terreran, G.
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Villar, V. A.
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Williams, P. K. G.
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Xie, X.
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Zrake, J.
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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., Laskar, T., Metzger, B. D., Nicholl, M., Paterson, K., Radice, D., Sironi, L., Terreran, G., Villar, V. A., Williams, P. K. G., Xie, X. and 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. The Astrophysical Journal Letters, 886 (1), L17. (doi:10.3847/2041-8213/ab5226).

Record type: Article

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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1909.06393 - Accepted Manuscript
Restricted to Repository staff only until 9 November 2020.
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Accepted/In Press date: 9 November 2019
e-pub ahead of print date: 18 November 2019

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Local EPrints ID: 436791
URI: http://eprints.soton.ac.uk/id/eprint/436791
ISSN: 2041-8205
PURE UUID: 7e989010-3381-42d9-a920-66e99aa0bc69

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Date deposited: 09 Jan 2020 17:30
Last modified: 15 Jan 2020 17:30

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Contributors

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

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