Bibliographic Details
| Title: |
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 |
| Authors: |
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., Zrake, J. |
| Publication Year: |
2019 |
| Collection: |
Astrophysics |
| Subject Terms: |
Astrophysics - High Energy Astrophysical Phenomena |
| More Details: |
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 $F_{\nu}\propto t^{-1.95\pm0.15}$ and a simple power-law spectrum $F_{\nu}\propto \nu^{-0.575\pm0.007}$. 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 $n\le 9.6 \times 10^{-3}\,\rm{cm^{-3}}$. 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 $E_0 = 1.5_{-1.1}^{+3.6}\times 10^{49}\,\rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}\times10^{52}\, \rm{erg}$), jet opening angle $\theta_{0}= 5.9^{+1.0}_{-0.7}\,\rm{deg}$ with characteristic Lorentz factor $\Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} \times 10^{-3}\, \rm{cm^{-3}}$ and viewed $\theta_{obs} = 30.4^{+4.0}_{-3.4}\, \rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are constrained to $\epsilon_{\rm{e}} = 0.18_{-0.13}^{+0.30}$ and $\epsilon_{\rm{B}}=2.3_{-2.2}^{+16.0} \times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\ge2.5\sigma$ 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 $E_k^{KN}\propto (\Gamma\beta)^{-\alpha}$ into the environment, finding that shallow stratification indexes $\alpha\le6$ are disfavored.
Comment: version accepted for publication in ApJL, 13 pages, 6 figures |
| Document Type: |
Working Paper |
| DOI: |
10.3847/2041-8213/ab5226 |
| Access URL: |
http://arxiv.org/abs/1909.06393 |
| Accession Number: |
edsarx.1909.06393 |
| Database: |
arXiv |
