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tokens: and afterglows can be used to constrain the angular structures of energy and Lorentz factor (Beniamini Nakar 2018 ), which are consistent with our results. In Figure 2 , we show the fitting results of the characteristic Lorentz factor (Γ ∼ 2 η 0 γ B ) for GW170817 and 14 short GRBs. GW170817 has Γ ≈ 150, which fits well within the range of short GRBs (Γ = 135 ± 47). For GW170817, the Lorentz factor long the line of sight to the observer is 5.3, which fits the expectation that the early emission is effectively dominated by radiation from mildly relativistic material. Figure 2. Fit results of the characteristic Lorentz factor Γ (left) and the spectral index p (right) for GW170817 (red circle) and 14 short GRBs (blue squares). Markers with error bars indicate median values and symmetric 68% quantiles. In Figure 2 , we show the fitting results of the spectral index p . GW170817 has a tight constraint , as it has good observational data from all three bands. The mean and standard deviation of short GRBs is p = 2.26 ± 0.22. The spectral index of GW170817 is consistent with those of short GRBs. Figures 3 shows the distributions of GW170817 and 14 short GRBs in the ( n 0,0 , E 0,50 ) plane. GW170817 has n 0 ≈ 10 −2 proton cm −3 and E 0 ≈ 6 × 10 49 erg. Considering the jet opening angle θ 0 ≈ 0.11 rad, the corresponding isotropic equivalent energy is E iso ≈ 8 × 10 52 erg. These values are located within the typical ranges of short GRBs. Because n 0,0 and E 0,50 are degenerate parameters, most of the bursts display large error bars. Due to the scaling relations, our model is better at constraining outflow structures rather than estimating accurate values of degenerate parameters. Even though these degenerate parameters are not well constrained, we note that their uncertainties can be marginalized out, and thus will not affect the constraints on non-degenerate parameters. In Wu MacFadyen ( 2018 ), we broke the degeneracies by fixing density as n 0,0 = 10 −3 , and the outflow structure of GW170817 remains consistent. The values of e for the short GRBs is found to be around 0.3, which is consistent with other studies (Cenko et al. 2010 ; Beniamini van der Horst 2017 ). Figure 3. Fit results of explosion energy E 0,50 and ISM density n 0,0 for 14 short GRBs (blue squares) and GW170817 (red circle). Markers with error bars indicate median values and symmetric 68% quantiles. 4.3. On-axis Light Curves We have found that GW170817 and short GRBs share similar outflow structures and other physical parameters, except for the observation angle. Viewed off-axis, GW170817 displayed exceptional behaviors, such as late onset and early brightening. Short GRBs are viewed on-axis and show a general decline shortly after prompt emission. This leads to an interesting question: what would be observed from GW170817 if the observer were located on-axis? In Figure 4 , we show the X-ray afterglow observations for GW170817 (red circles with error bars) and 27 cosmological short