tokens: groups as an extra fractional uncertainty, f , on all data points. 10 10 We use this statistically robust approach instead of assuming a fixed fractional uncertainty of 3%–5% at each frequency as done by Dobie et al. ( 2018 ). We use emcee (Foreman-Mackey et al. 2013 ) to perform a MCMC analysis to determine the posteriors of the model parameters. We use a logarithmic flat prior on s (−1.5 log s 1.5) because by definition s must be positive and the data do not provide any constraints once s becomes sufficiently large (corresponding to a sharp break that occurs over a time interval much smaller than our sampling time). We use flat linear priors on all other parameters: 10 t b 1000 days, 1 F ν , b 1000 μ Jy, −20 α 1 , α 2 20, and 0 f 1. Figure 1. Up-to-date X-ray, optical, and radio light curves of GW170817 (solid circles; open circles are the new data presented in Dobie et al. 2018 ). The data are clearly indicative of a decline at ≳200 days. Also shown are our structured jet models from Margutti et al. ( 2018 ); see Xie et al. ( 2018 ) for full details of the simulations. Both jets have an ultra-relativistic core with E K,iso = 6 ×10 52 erg within an opening angle θ jet = 9°. The solid lines are for a model with n = 10 −5 cm −3 , θ obs = 17°, e = 0.1, and B = 0.0005, while the dashed lines are for n = 10 −4 cm −3 , θ obs = 20°, e = 0.02, and B = 0.001. Our new radio, optical, and X-ray observations continue to support these models. We find and , with days, indicative of a clear transition from a rising to a declining light curve; see Figure 2 . We can reject a non-declining light curve (i.e., α 2 ≥ 0) at 7 σ confidence level. These results are consistent 11 11 We repeat our MCMC analysis for the subset of radio data presented in Dobie et al. ( 2018 ), with α 1 , α 2 , t b , F ν , b , s , and f as free parameters, and find that t b and α 2 are more loosely constrained: t b = 190 ± 30 days and . A value of α 2 ≥ 0 can be ruled out at 3.0 σ confidence level. with the analysis of Dobie et al. ( 2018 ), but provide a much stronger indication of a break due to the longer time baseline of our observations. Both successful structured jet models and pure cocoon models are consistent with the observed α 1 . Unlike the simplest analytic models, which predict α 2 ≈ −2.2 (Sari et al. 1999 ; Nakar Piran 2018 ), the Xie et al. ( 2018 ) successful structured jet simulations predict a shallower initial post-peak decline (Figure 1 ). Therefore, our derived α 2 does not strongly discriminate between successful structured jet and pure cocoon or dynamical ejecta models, although the latter are mildly disfavored (Lamb et al. 2018 ). Of the two Xie et al. ( 2018 ) models shown in Figure 1 , our most recent data at 273 days and 289 days mildly favor the dashed model, which in comparison to the solid model implies a higher circumbinary density of 10 −4 cm −3 , a lower fraction of the shock energy imparted to electrons ( e = 0.02), and an observer viewing angle
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